U.S. Senate

Why this race matters

This is the 31st closest Senate race in the forecast, 24.5 points, out of 35.

The published margin is the leader over the next candidate. The opponent is not the other major party, so this race is not treated as a Republican-minus-Democrat swing.

35 Senate races are on the 2026 ballot in this forecast (15 Democratic leads, 20 Republican leads). Seats that are not on the ballot are not assigned, so this is not a 100-seat chamber.

No earlier forecast of this race is archived, so there is no previous projection, date, or reason to show.

History block in the model: R+35.7. That is the office blend, not one past election.

Last same-office result in the candidate file: 2022 R+43.5. A special on the 2026 ballot is not automatically that contest.

The polling block for this race was omitted.

Counties

Every reporting unit in the race. Tap a county to open its precincts. The projected votes above are this table added up. They are not ballots cast.

Counties

County	Leader	Party	Margin	Vote share	Projected ↓
Minnehaha County	RMike Rounds, RMike Rounds	R	R+9.1	53.4%	76,659
Pennington County	RMike Rounds, RMike Rounds	R	R+22.8	60.2%	46,149
Lincoln County	RMike Rounds, RMike Rounds	R	R+22.9	60.3%	29,683
Brown County	RMike Rounds, RMike Rounds	R	R+23.2	60.4%	13,901
Lawrence County	RMike Rounds, RMike Rounds	R	R+27.7	62.7%	12,568
Brookings County	RMike Rounds, RMike Rounds	R	R+13.8	55.7%	12,185
Meade County	RMike Rounds, RMike Rounds	R	R+47.3	72.5%	11,968
Codington County	RMike Rounds, RMike Rounds	R	R+37.2	67.4%	11,019
Yankton County	RMike Rounds, RMike Rounds	R	R+21.9	59.8%	8,835
Davison County	RMike Rounds, RMike Rounds	R	R+34.1	65.9%	7,486
Union County	RMike Rounds, RMike Rounds	R	R+37.1	67.4%	7,240
Hughes County	RMike Rounds, RMike Rounds	R	R+26.5	62.1%	6,880
Beadle County	RMike Rounds, RMike Rounds	R	R+36.4	67.1%	5,718
Custer County	RMike Rounds, RMike Rounds	R	R+42.1	69.9%	4,895
Lake County	RMike Rounds, RMike Rounds	R	R+27.5	62.6%	4,817
Clay County	IBrian Bengs, IBrian Bengs	I	+10.1	53.9%	4,614
Butte County	RMike Rounds, RMike Rounds	R	R+57.0	77.4%	4,146
Turner County	RMike Rounds, RMike Rounds	R	R+47.7	72.7%	3,701
Fall River County	RMike Rounds, RMike Rounds	R	R+45.6	71.7%	3,486
Roberts County	RMike Rounds, RMike Rounds	R	R+19.4	58.5%	3,384
Hutchinson County	RMike Rounds, RMike Rounds	R	R+54.3	76.0%	3,047
Grant County	RMike Rounds, RMike Rounds	R	R+41.8	69.7%	2,961
Charles Mix County	RMike Rounds, RMike Rounds	R	R+39.3	68.5%	2,949
Hamlin County	RMike Rounds, RMike Rounds	R	R+56.8	77.2%	2,631
Moody County	RMike Rounds, RMike Rounds	R	R+28.2	62.9%	2,601
Spink County	RMike Rounds, RMike Rounds	R	R+35.4	66.5%	2,560
Oglala Lakota County	IBrian Bengs, IBrian Bengs	I	+74.2	85.9%	2,498
McCook County	RMike Rounds, RMike Rounds	R	R+45.7	71.7%	2,472
Bon Homme County	RMike Rounds, RMike Rounds	R	R+47.8	72.8%	2,441
Day County	RMike Rounds, RMike Rounds	R	R+26.3	62.0%	2,392
Kingsbury County	RMike Rounds, RMike Rounds	R	R+40.3	69.0%	2,285
Tripp County	RMike Rounds, RMike Rounds	R	R+59.7	78.7%	2,165
Walworth County	RMike Rounds, RMike Rounds	R	R+48.7	73.2%	1,458
Brule County	RMike Rounds, RMike Rounds	R	R+31.9	64.8%	1,432
Deuel County	RMike Rounds, RMike Rounds	R	R+41.3	69.5%	1,359
Gregory County	RMike Rounds, RMike Rounds	R	R+49.7	73.7%	1,341
Todd County	IBrian Bengs, IBrian Bengs	I	+60.9	79.3%	1,257
Marshall County	RMike Rounds, RMike Rounds	R	R+13.5	55.6%	1,247
Dewey County	IBrian Bengs, IBrian Bengs	I	+21.1	59.3%	1,220
Hanson County	RMike Rounds, RMike Rounds	R	R+48.8	73.2%	1,210
Edmunds County	RMike Rounds, RMike Rounds	R	R+50.2	73.9%	1,201
Clark County	RMike Rounds, RMike Rounds	R	R+42.0	69.8%	1,089
Hand County	RMike Rounds, RMike Rounds	R	R+46.8	72.2%	1,043
Stanley County	RMike Rounds, RMike Rounds	R	R+36.3	67.0%	1,026
Douglas County	RMike Rounds, RMike Rounds	R	R+61.8	79.7%	981
Perkins County	RMike Rounds, RMike Rounds	R	R+59.4	78.5%	941
Lyman County	RMike Rounds, RMike Rounds	R	R+28.9	63.3%	855
Aurora County	RMike Rounds, RMike Rounds	R	R+43.6	70.6%	824
McPherson County	RMike Rounds, RMike Rounds	R	R+58.6	78.1%	768
Potter County	RMike Rounds, RMike Rounds	R	R+54.7	76.2%	768
Corson County	RMike Rounds, RMike Rounds	R	R+3.6	50.7%	746
Sanborn County	RMike Rounds, RMike Rounds	R	R+44.6	71.1%	717
Miner County	RMike Rounds, RMike Rounds	R	R+36.5	67.1%	690
Faulk County	RMike Rounds, RMike Rounds	R	R+54.8	76.2%	668
Jackson County	RMike Rounds, RMike Rounds	R	R+24.6	61.1%	668
Haakon County	RMike Rounds, RMike Rounds	R	R+69.4	83.5%	666
Bennett County	RMike Rounds, RMike Rounds	R	R+15.9	56.8%	643
Jerauld County	RMike Rounds, RMike Rounds	R	R+32.5	65.1%	590
Sully County	RMike Rounds, RMike Rounds	R	R+50.3	73.9%	533
Ziebach County	IBrian Bengs, IBrian Bengs	I	+5.5	51.6%	510
Campbell County	RMike Rounds, RMike Rounds	R	R+59.5	78.5%	494
Harding County	RMike Rounds, RMike Rounds	R	R+70.9	84.3%	485
Mellette County	RMike Rounds, RMike Rounds	R	R+9.4	53.5%	437
Hyde County	RMike Rounds, RMike Rounds	R	R+44.5	71.0%	411
Jones County	RMike Rounds, RMike Rounds	R	R+65.2	81.5%	325
Buffalo County	IBrian Bengs, IBrian Bengs	I	+35.6	66.5%	275

Model

1. Raw data

   2024 presidential baseline

   National R−D 1.5

   County presidential file. Not the 2026 result.

2. Raw data

   Generic ballot

   D 51.7 R 43.1 R−D -8.6

   National Wikipedia aggregate, 2026-10-30. State generic ballots, where a public poll exists, are mixed with the national swing for a race that does not have its own poll: ME, MI, NH, NY, OH. A state with no generic-ballot poll keeps the national swing and is not assigned a neighbor's.

3. Raw data

   Party identification

   Pew D+5 and Gallup D+10, blend R−D -7.3

   Pew NPORS 2026, identifiers plus leaners: Republican 43, Democratic 48. Gallup party identification plus leaners, first two quarters of 2026: Democratic 49, Republican 39. Pew is 55% of this blend and Gallup is 45%.

4. Model input

   National block

   R−D -8.1

   60% generic ballot and 40% party identification. The swing versus the 2024 presidential margin is what gets transmitted.

5. Model input

   Swing transmitted

   1.30 × -10.1 = -13.1

   No state generic-ballot poll. The generic ballot is R−D -8.6, a swing of -10.1 from the 2024 presidential margin. A race without its own poll uses that swing, not the 60/40 block. This state is not given another state's number.

6. Model input

   History block

   R−D 35.7

   Weights 2018 5 / 2024 35 / 2020 20 / 2022 40. If that office was on the ballot in 2022, that year takes the 40 and 2018 takes the 5. Otherwise 2018 keeps the 40. A missing year is filled with the 2020 result for the same office. Another office is not used. The year lines are Minnehaha County, the largest county, not the state total: 2018 from 2020 R+15.0 w5, 2024 from 2020 R+15.0 w35, 2020 R+15.0 w20, 2022 R+27.0 w40. The state number above is the vote-weighted sum. A 2024 county map is not copied forward unchanged.

7. Model input

   Electorate block

   R−D 29.4

   History reweighted by usual midterm turnout, plus half of the transmitted swing. Rural and presidential-only counties still move farther when the result is spread back to counties. That spread is ballot size, not the demographic file. Published ballots are the 2018 midterm canvass for this office, split by these modeled shares. They are not the 2024 presidential vote. The 2026 early-vote count is not inside this margin.

8. Model input

   Turnout block

   R−D 35.9

   Each county is weighted by its 2018 ballots divided by its 2024 presidential ballots. Small counties shrink halfway toward the state median. 2020 mail shares are not used.

9. Model input

   National environment

   R−D 22.6

   History plus the full transmitted swing. With no poll of this race, that swing is the state generic ballot mixed with the national block, or the national block alone.

10. Model input

    State environment

    R−D 29.9

    History plus 45 percent of the transmitted swing. This is a Senate and governor weight.

11. Model input

    State partisan lean

    R−D 29.2

    The 2024 presidential margin of this state, or of this district for a House race. It is heavier than the old state-environment block and is not shrunk toward the polling term.

12. Model input

    Trump popularity

    R−D -4.1

    New York Times approval average, October 29, 2026: 37.4% approve, 60.2% disapprove. Net approval -22.800000000000004. That is not a vote share. Four-tenths of the net is a margin, then scaled by this place's partisan lean (factor 0.45), so an unpopular president costs more in a Democratic state.

13. Model input

    Candidate favorability

    R−D 0.0

    Mike Rounds's 2020 race is already in the history block, so it is not applied again as favorability.

14. Model input

    Demographics

    R−D 19.5

    One statewide block only. A county that is more college-educated or more nonwhite than the other counties in this race takes more of the transmitted swing, and a more rural county takes less. That changes this block, at its weight. It does not choose which counties move when the margin is spread back out. That spread uses ballot size. The shares are the 2012–2016 American Community Survey and the 2010 Census, from the 2018 county election-context file. Alaska is not in that file.

15. Model input

    Finance

    R−D 35.7

    No sourced spending edge. The block is history, or history shrunk toward the poll. It is not a finance model.

16. Model input

    Issues

    R−D 35.7

    No sourced issue score. Same treatment as finance. It is not a separate issue model.

17. Model input

    Polling

    Omitted

    No public poll. Rated from the generic congressional ballot, R−D -8.6. No state generic-ballot poll. The generic ballot is R−D -8.6, a swing of -10.1 from the 2024 presidential margin. A race without its own poll uses that swing, not the 60/40 block. This state is not given another state's number. There is no public poll, so there is no undecided line to split. Banned individual rows excluded: 0. No same-office polling miss was sourced, so none is added. Applied to the polling term: 0.00. Public pollsters: 0. Outliers: 0. Recency half-life 16 days. Sample weight n/(n+350). A registered-voter poll is 0.90. An internal poll, or a poll sponsored by one party, is weighted 0. It is not in the average. An internal-only file is dropped. A poll that names both parties is not an internal. A poll more than 8 points from the median of the others is cut in half. Pollster quality nudges that weight by only a few percent. The same rule is used on a House poll. House district polls are likely-voter polls only.

18. Model input

    Generic ballot weight

    R−D 22.6

    This race has no independent poll, so the generic ballot is already the environment swing. It is not added a second time.

19. Model input

    Top line

    Not a House term

    Senate and governor are the top line. They are not moved by themselves.

20. Model input

    Candidate

    R+2.30 added whole

    Mike Rounds keeps the incumbent term of 2.3 after shrinkage. Overperformance is a separate term and is not added again here. Mike Rounds has held this office about 12 years, so 0.8 is added for long service. Under eight years is unchanged. Mike Rounds's 2020 race is already in the history block, so it is not applied again. These points are not watered down to the candidate weight. The candidate weight is a separate slice of the baseline. On a Senate or governor race with three public polls and no outlier, the points stop at that poll instead of moving past it.

21. Model input

    Scandal

    None

    No candidate on this ballot is scandal-defined.

22. Model input

    Endorsements

    R−D +0.30

    Trump endorsed Mike Rounds. The presidential lean here is R−D 29.2, so the endorsement helps 0.30. Ballotpedia, Endorsements by Donald Trump, 2026 tables, page updated August 5, 2026. A primary endorsement is counted when that person is the nominee. A lost primary, a candidate not on the ballot, and a withdrawn endorsement are not.

23. Model input

    Halfway floor

    Not applied

    Not applied. The state's 2024 presidential margin was inside 10 points, or there is no polling term.

24. Model input

    Pooling

    No poll to shrink toward

    Otherwise the shrink is 35%.

25. Model input

    Prediction market

    95% win probability

    Mike Rounds 95.1% is a win probability, not a vote share. Republican win probability 95.1 from Kalshi 95.0% Republican; Polymarket 95.2% Republican. A price of 50 is a tie. Each point above 50 is 0.35 margin points, capped at 12, so the raw margin is 12.0. That margin is shrunk 40% toward the pre-market margin 23.7, which leaves 16.7. It enters once, at 5.0%. The polling term is not nudged. Books under $10,000 and primary markets are ignored.

26. Model input

    State bend

    None

    Not applied.

27. Model input

    State environment adjustment

    R−D -0.6

    The 2022 statewide margin was R−D 26.8 and the 2024 presidential margin was R−D 29.2. The country moved 1.4 from that presidential vote to the 2022 House vote. Fifteen percent of the difference is kept, capped at 1.5, so the state environment is -0.6 on top of the national swing.

28. Model input

    Early vote

    D+0.1

    Party is on the file, but there is no 2022 same-day comparison, so the raw lead is not the adjustment. Ballots that match a 2024 presidential county sit at R−D 28.7. The 2024 vote in the same counties was R−D 29.2. The early mix is D+0.5 relative to that vote. Fifteen percent is kept, capped at 1, so D+0.1. This is where the ballots were cast. A more college-educated or more nonwhite county still takes more of the national swing in the demographics block, and a more rural county takes less. That block is separate from this point. This is not a count of how anyone voted, and not another model's estimate of the electorate.

29. Model input

    Other ratings

    R−D 11.0 at 3.0%

    Cook, Sabato, Inside Elections, and the other columns on the same published tables (Decision Desk HQ, RealClearPolitics, Split Ticket, FiftyPlusOne, The Economist, Fox News, Silver Bulletin). 10 ratings, average R−D 11.0. A toss-up is 0, tilt 1.5, lean 3.5, likely 7, safe or solid 12. If this race is already further to that side, the rating does not pull it back. Mixed in after everything else, at 3.0 percent. It does not replace the poll. A seat nobody has rated is left alone.

30. Model input

    Weights

    stateLean 20.7% · trump 13.8% · favorability 12.7% · history 9.2% · candidate 6.9% · national 6.9% · electorate 5.7% · stateEnv 4.6% · turnout 3.4% · demographics 3.4% · finance 2.3% · issues 2.4% · market 4.9% · ratings 3.0%

    History is lighter. State partisan lean, Trump popularity, and candidate favorability are their own terms. A market, when it qualifies, scales every other weight and is not taken out of polling alone. Published ratings, when a race has them, are mixed in after that. They are not taken out of polling alone.

31. County-sum margin

    24.5

    Leader over the next candidate. The opponent is not the other major party, so that party's share is not zero. Mike Rounds leads Brian Bengs by 24.5 points.

32. Minor parties

    None

    Other is the 2024 presidential vote that was neither Democratic nor Republican, weighted by these counties. It is not the last forecast. A missing opponent is not Other. No Libertarian, Green, Constitution, or other minor line was on this ballot.

33. 80% interval

    14.8 to 34.2

    10th to 90th percentile of 50,000 draws. Poll, national, state, turnout, and candidate error.

34. 95% interval

    9.7 to 39.3

    2.5th to 97.5th percentile. Senate and governor state errors in the same state share a correlation of 0.30. The band shown is the marginal interval.