Which Republican Senators Are Most Likely To Fight Trump? And which Democrats are most likely to work with him?

Out of the rubble of the Nov. 8 election, Democrats managed to pick up two U.S. Senate seats from Republicans — in Illinois and New Hampshire — while holding on to all of their own. It was an underwhelming performance given how many chances Democrats had to gain seats all over the map. Still, Donald Trump will begin his presidential term with Republicans holding only a narrow Senate majority: either 51-49 or (more likely) 52-48, depending on the results of Louisiana’s runoff election on Dec. 10.

That means defections of just two to three Republican senators could block Trump from confirming his cabinet, making successful Supreme Court appointments, or passing new pieces of legislation. But how many anti-Trump Republicans can we really expect in the Senate, especially given that all but three Republican senators come from states that voted for him? And with the daunting Senate map that Democrats face in 2018 — with several red-state Democrats up for re-election — how many of their own senators might join with Trump on key votes?

Until the Senate convenes on Jan. 3, the best we can do is make educated guesses about the coalitions that will line up around Trump. But there are a few obvious factors that will presumably correlate with support for his agenda:

Issue alignment: Senators whose policy positions are more aligned with Trump’s will be more likely to support him.

Senators whose policy positions are more aligned with Trump’s will be more likely to support him. Personal support: Senators who endorsed Trump — and there were a lot of gradations of endorsements and non-endorsements — will be more likely to support him.

Senators who endorsed Trump — and there were a lot of gradations of endorsements and non-endorsements — will be more likely to support him. Electoral incentive: The better Trump performed in a senator’s state, the more likely the senator will be to support Trump, other factors held equal, especially if the senator is up for re-election in 2018.

I came up with a quick way to measure each of these qualities on a 1-to-5 scale, where 5 implies a higher likelihood of supporting Trump. Although the calculation for the first factor, issue alignment, is fairly involved, this is not meant to be a terribly precise exercise. Instead, we just want to get a rough sense for how many potential Republican defectors or Democratic supporters there might be for Trump on contentious votes. Not to be overlooked: How easy or challenging it will be for Trump to gather the 60 votes necessary to overcome potential Democratic filibusters, a tactic that probably won’t be going away anytime soon.

Here’s a brief description of how I calculated the score for each factor:

Issue alignment. Trump was an unusual Republican nominee in that he didn’t have down-the-line conservative views, instead mixing extremely conservative stances on issues such as immigration with surprisingly moderate (or even leftist) ones on other issues such as trade — with a lot of improvisation (and inconsistency) along the way. Therefore, the usual statistical methods that rate members of Congress on a one-dimensional, liberal-conservative scale may not be the best way to predict compatibility with Trump.

Instead, I used data from OnTheIssues.org, which tracks candidates’ positions within 20 policy areas based on their public statements and voting history. The site rates candidates’ positions on a 10-point scale from -5 to +5 — for instance, on immigration, -5 represents a conservative stance (start deporting people) whereas +5 is a liberal one (amnesty for immigrants who entered the country illegally).

We can calculate each senator’s similarity to Trump in the aggregate over these various issues. For each issue, I measured the distance between each senator’s stance and Trump’s position on the 10-point scale. Then I squared the result, which punishes larger differences of opinion as compared to smaller ones, and weighted each issue based on how often Trump talks about it — so immigration matters more than school choice, for instance. This calculation resulted in a weighted, root-mean-squared distance (RMSD) from Trump, where lower numbers meant more similarity to him.

REPUBLICANS DEMOCRATS SENATOR STATE SCORE SENATOR STATE SCORE 1 Perdue GA 2.6 37 Heitkamp ND 4.1 2 Scott SC 3.1 — Campbell* LA 4.7 3 Risch ID 3.4 52 Warner VA 4.8 4 Sasse NE 3.4 54 Donnelly IN 5.3 5 Tillis NC 3.4 55 Manchin WV 5.3 6 Cassidy LA 3.5 56 McCaskill MO 5.7 7 Shelby AL 3.6 58 Hassan NH 5.8 8 Enzi WY 3.6 59 Merkley OR 5.9 9 Sullivan AK 3.6 60 Tester MT 5.9 10 Gardner CO 3.6 61 Klobuchar MN 6.1 12 Johnson WI 3.6 62 Shaheen NH 6.1 12 Alexander TN 3.6 63 Schatz HI 6.1 13 Young IN 3.6 64 King ME 6.1 14 Barrasso WY 3.6 65 Casey PA 6.2 15 Cruz TX 3.7 66 Carper DE 6.3 16 Rubio FL 3.7 67 Booker NJ 6.4 17 Ernst IA 3.8 68 Stabenow MI 6.4 18 Toomey PA 3.8 69 Kaine VA 6.4 19 Daines MT 3.8 70 Coons DE 6.5 20 Inhofe OK 3.8 71 Heinrich NM 6.5 21 Fischer NE 3.8 72 Cortez Masto NV 6.6 22 Graham SC 3.8 73 Blumenthal CT 6.6 23 Burr NC 3.9 74 Wyden OR 6.7 24 Hoeven ND 3.9 75 Murphy CT 6.7 25 Crapo ID 3.9 76 Peters MI 6.8 26 Sessions AL 3.9 77 Gillibrand NY 6.8 27 Cochran MS 3.9 78 Feinstein CA 6.9 28 Flake AZ 4.0 79 Harris CA 7.0 29 Capito WV 4.1 80 Bennet CO 7.0 30 Corker TN 4.1 81 Nelson FL 7.0 31 Isakson GA 4.1 82 Brown OH 7.0 32 Cotton AR 4.1 83 Menendez NJ 7.1 33 Thune SD 4.1 84 Sanders VT 7.1 34 Cornyn TX 4.1 85 Leahy VT 7.1 35 Boozman AR 4.1 86 Durbin IL 7.1 36 Lankford OK 4.1 87 Reed RI 7.2 38 Wicker MS 4.2 88 Schumer NY 7.2 39 Hatch UT 4.2 89 Franken MN 7.2 40 Lee UT 4.2 90 Hirono HI 7.2 41 Moran KS 4.3 91 Warren MA 7.2 42 McConnell KY 4.3 92 Whitehouse RI 7.3 43 Grassley IA 4.4 93 Udall NM 7.3 44 Roberts KS 4.4 94 Markey MA 7.4 45 Heller NV 4.5 95 Cardin MD 7.4 46 Kennedy* LA 4.5 96 Duckworth IL 7.5 47 Blunt MO 4.6 97 Baldwin WI 7.5 48 Rounds SD 4.6 98 Van Hollen MD 7.5 49 Murkowski AK 4.7 99 Cantwell WA 7.6 50 Portman OH 4.8 100 Murray WA 7.7 51 Paul KY 4.8 53 McCain AZ 5.0 57 Collins ME 5.8 Which senators are most similar to Trump? *Either Campbell or Kennedy will be elected in the Louisiana runoff on Dec. 10. Source: ontheissues.org

Because of Trump’s unorthodox stances, no senator is a carbon copy of him. (Perhaps that will change once Trump has been in office four years and reshapes the party in his image.) The closest, according to our similarity score, is Sen. David Perdue of Georgia, who like Trump was a wealthy businessman before running for office. And in general, senators who were first elected during the tea party era from 2010 onward are more similar to Trump than those who have held longer tenures in office. The former group includes some senators, such as Ben Sasse of Nebraska, who have publicly feuded with Trump. Their beefs with Trump may have less to do with differences over policy and more about his style of leadership and fitness for office.

By contrast, a few Republican senators really don’t have much in common with Trump, policywise. Sen. Susan Collins of Maine, especially, has more differences with Trump than similarities. Arizona Sen. John McCain and Trump may both be “mavericks,” but not in a way that will necessarily be compatible with one another. Kentucky Sen. Rand Paul’s libertarian-ish views are a bad mix for Trump’s populist ones. And moderate, pro-free-market Republicans such as Ohio Sen. Rob Portman may find themselves at odds with Trump on issues such as trade.

Among Democrats, Trump has some commonalities with moderate, populist Democrats such as North Dakota Sen. Heidi Heitkamp, Indiana Sen. Joe Donnelly and West Virginia Sen. Joe Manchin. Heitkamp, in particular, rates as being closer to Trump on the issues than many Republicans. Several of these Democrats also happen to be up for re-election in 2018 in states that Trump won by wide margins (more about that in a moment). Meanwhile, the least similar senators to Trump are Patty Murray and Maria Cantwell of Washington, liberal Democrats who (unlike many of their liberal compatriots) support free trade.

(Note: for easier comparison with the other metrics, I translated the RMSD scores to a 5-point scale, where 5 indicates more similarity to Trump. You can find that calculation in the footnotes. )

Personal support. I also rated the senators on a 5-point scale based on whether, when and how they endorsed Trump. (The primary sources are Daniel Nichanian’s spreadsheet and FiveThirtyEight’s own endorsement tracker from the primaries). The rating scale is as follows, ranging from least supportive of Trump to the most.

1 point — endorsed Hillary Clinton. All current and newly elected Democratic senators fall into this category, except for Foster Campbell, the Democrats’ candidate in the Louisiana runoff, who didn’t endorse either Clinton or Trump.

All current and newly elected Democratic senators fall into this category, except for Foster Campbell, the Democrats’ candidate in the Louisiana runoff, who didn’t endorse either Clinton or Trump. 2 points — endorsed Evan McMullin or Gary Johnson . This category applies only to Sen. Lindsey Graham of South Carolina, who endorsed and then voted for McMullin.

. This category applies only to Sen. Lindsey Graham of South Carolina, who endorsed and then voted for McMullin. 3 points — didn’t endorse Trump or anyone else. Several Republicans, such as Collins, fall into this category, including some like McCain who unendorsed Trump after the “Access Hollywood” tape surfaced.

Several Republicans, such as Collins, fall into this category, including some like McCain who unendorsed Trump after the “Access Hollywood” tape surfaced. 3.5 points — ambiguous on Trump . Sen. Shelley Moore Capito of West Virginia called on Trump to “re-examine his candidacy” after the “Access Hollywood” tape, but never formally unendorsed him, nor did she ever reaffirm her original endorsement.

. Sen. Shelley Moore Capito of West Virginia called on Trump to “re-examine his candidacy” after the “Access Hollywood” tape, but never formally unendorsed him, nor did she ever reaffirm her original endorsement. 4 points — wavered, then endorsed Trump . These candidates eventually endorsed Trump by Election Day, but either unendorsed him at some point during the campaign before re-endorsing him, endorsed one of his opponents in the primary or ran against him in the primary.

. These candidates eventually endorsed Trump by Election Day, but either unendorsed him at some point during the campaign before re-endorsing him, endorsed one of his opponents in the primary or ran against him in the primary. 5 points — endorsed Trump without wavering. Everyone else falls into this category.

Electoral incentive. Only three Republican senators — Collins of Maine, Cory Gardner of Colorado and Dean Heller of Nevada — came from states that Trump lost last week. In fact, many Republicans probably have more reason to worry about a primary challenge than their general election prospects. That could help keep Sasse in line, for example.

By contrast, 11 or 12 Senate Democrats will hail from states won by Trump, depending on whether Campbell wins the Louisiana runoff. But there are obviously varying degrees of Trumpiness in their states. It looks as if Trump will eventually win Michigan by a couple 10ths of a percentage point — results aren’t finalized yet — but Michigan Sen. Debbie Stabenow doesn’t have as much to worry about as Heitkamp, given that Trump won North Dakota by 36 percentage points. I translated Trump’s margin of victory or defeat in each state onto a 5-point scale, as detailed in the footnotes.

Calculating the Trump support score. Finally, I estimated each senator’s overall likelihood to support Trump by averaging the three categories, but with a twist: I put more weight on the state’s electoral environment if the senator is up for re-election soon. In particular, I gave the electoral incentive category 1.5 times the weight of the other categories if the senator is up for re-election in 2018, the same weight as the other categories if he or she is up in 2020, and a weight of 0.5 if he or she was just elected and isn’t on the ballot again until 2022.

Here are the scores for the 51 or 52 Republican senators, ranked in inverse order of their Trump support score.

SENATOR STATE RE-ELECT YEAR ISSUE ALIGNMENT PERSONAL SUPPORT ELECTORAL INCENTIVE TRUMP SUPPORT SCORE Collins ME 2020 2.5 3 2.5 2.7 Heller NV 2018 3.5 3 2.5 2.9 McCain AZ 2022 3 3 3.5 3.1 Gardner CO 2020 4 3 2.5 3.2 Portman OH 2022 3 3 4 3.2 Murkowski AK 2022 3 3 4.5 3.3 Flake AZ 2018 3.5 3 3.5 3.4 Graham SC 2020 4 2 4.5 3.5 Lee UT 2022 3.5 3 4.5 3.5 Toomey PA 2022 4 4 3 3.8 Paul KY 2022 3 4 5 3.8 Rubio FL 2022 4 4 3 3.8 Sullivan AK 2020 4 3 4.5 3.8 Capito WV 2020 3.5 3.5 5 4.0 Rounds SD 2020 3 4 5 4.0 Cruz TX 2018 4 4 4 4.0 Boozman AR 2022 3.5 4 5 4.0 Thune SD 2022 3.5 4 5 4.0 Tillis NC 2020 4.5 4 3.5 4.0 Hatch UT 2018 3.5 4 4.5 4.1 Blunt MO 2022 3 5 4.5 4.1 Kennedy* LA 2022 3 5 4.5 4.1 Sasse NE 2020 4.5 3 5 4.2 Roberts KS 2020 3.5 4 5 4.2 McConnell KY 2020 3.5 4 5 4.2 Cochran MS 2020 4 4 4.5 4.2 Cornyn TX 2020 3.5 5 4 4.2 Grassley IA 2022 3.5 5 4 4.2 Isakson GA 2022 3.5 5 4 4.2 Johnson WI 2022 4 5 3 4.2 Crapo ID 2022 4 4 5 4.2 Burr NC 2022 4 5 3.5 4.3 Scott SC 2022 4.5 4 4.5 4.3 Daines MT 2020 4 4 5 4.3 Alexander TN 2020 4 4 5 4.3 Inhofe OK 2020 4 4 5 4.3 Ernst IA 2020 4 5 4 4.3 Wicker MS 2018 3.5 5 4.5 4.4 Moran KS 2022 3.5 5 5 4.4 Lankford OK 2022 3.5 5 5 4.4 Fischer NE 2018 4 4 5 4.4 Cotton AR 2020 3.5 5 5 4.5 Risch ID 2020 4.5 4 5 4.5 Young IN 2022 4 5 4.5 4.5 Corker TN 2018 3.5 5 5 4.6 Hoeven ND 2022 4 5 5 4.6 Shelby AL 2022 4 5 5 4.6 Sessions AL 2020 4 5 5 4.7 Enzi WY 2020 4 5 5 4.7 Cassidy LA 2020 4.5 5 4.5 4.7 Perdue GA 2020 5 5 4 4.7 Barrasso WY 2018 4 5 5 4.7 Which Republicans might defy Trump? *Kennedy’s position is pending results of Dec. 10 runoff in Louisiana Sources: OnTheIssues.org, Daniel Nichanian, David Wasserman

Collins, although she’s reportedly set to back Trump’s nomination of Jeff Sessions as Attorney General, is a good bet to be the most frequent Republican vote against Trump, not having much in common with him on the issues, never having endorsed him, and hailing from a blue state — although Maine was much closer than usual this year. Still, her vote alone wouldn’t be enough to block Trump.

Instead, there’s a set of five to 10 Republican senators who might defect from Trump, depending on the circumstances. One group consists of Republicans who might be on thin ice with swing voters if Trump proves to be an unpopular president. Heller — the only Republican from a Clinton state up for re-election in 2018 — probably faces the most acute electoral pressure. Democrats will also almost certainly make a serious play for Arizona Sen. Jeff Flake’s seat in 2018 given that Arizona is trending blue and they don’t have very many alternatives, and Flake was among the more consistent Republican critics of Trump over the course of the campaign. And Gardner will face a potential toss-up race in 2020.

On the flip side are Republicans who might be free to defy Trump because they don’t face much electoral pressure. McCain, who just won re-election in Arizona, was a thorn in the side of George W. Bush early in his first term and could do the same to Trump. Portman, Paul, Alaska Sen. Lisa Murkowski and Utah Sen. Mike Lee also won re-election last week by commanding margins and could afford to defy Trump if they have a philosophical disagreement with him. Graham, up for re-election in 2020 and never having been all that popular with the Republican base to begin with, is in a somewhat riskier position.

But if it won’t always be that easy for Trump to rustle up support from 50 Republicans, there are also times when he can probably win votes from some Democrats:

SENATOR STATE RE-ELECT YEAR ISSUE ALIGNMENT PERSONAL SUPPORT ELECTORAL INCENTIVE TRUMP SUPPORT SCORE Heitkamp ND 2018 3.5 1 5 3.4 Campbell* LA 2022 3 3 4.5 3.3 Manchin WV 2018 3 1 5 3.3 Tester MT 2018 2.5 1 5 3.1 Donnelly IN 2018 3 1 4.5 3.1 McCaskill MO 2018 2.5 1 4.5 2.9 Brown OH 2018 1 1 4 2.3 Stabenow MI 2018 2 1 3 2.1 Klobuchar MN 2018 2 1 3 2.1 Casey PA 2018 2 1 3 2.1 Hassan NH 2022 2.5 1 3 2.0 Warner VA 2020 3 1 2 2.0 Shaheen NH 2020 2 1 3 2.0 King ME 2018 2 1 2.5 1.9 Baldwin WI 2018 1 1 3 1.9 Nelson FL 2018 1 1 3 1.9 Peters MI 2020 1.5 1 3 1.8 Kaine VA 2018 2 1 2 1.7 Merkley OR 2020 2.5 1 1.5 1.7 Franken MN 2020 1 1 3 1.7 Heinrich NM 2018 1.5 1 2 1.6 Bennet CO 2022 1.5 1 2.5 1.5 Cortez Masto NV 2022 1.5 1 2.5 1.5 Booker NJ 2020 2 1 1.5 1.5 Coons DE 2020 2 1 1.5 1.5 Carper DE 2018 2 1 1.5 1.5 Murphy CT 2018 1.5 1 1.5 1.4 Schatz HI 2020 2 1 1 1.3 Udall NM 2020 1 1 2 1.3 Blumenthal CT 2022 1.5 1 1.5 1.3 Wyden OR 2022 1.5 1 1.5 1.3 Cantwell WA 2018 1 1 1.5 1.2 Whitehouse RI 2018 1 1 1.5 1.2 Menendez NJ 2018 1 1 1.5 1.2 Harris CA 2022 1.5 1 1 1.2 Durbin IL 2020 1 1 1.5 1.2 Reed RI 2020 1 1 1.5 1.2 Feinstein CA 2018 1.5 1 1 1.1 Gillibrand NY 2018 1.5 1 1 1.1 Duckworth IL 2022 1 1 1.5 1.1 Murray WA 2022 1 1 1.5 1.1 Van Hollen MD 2022 1 1 1 1.0 Hirono HI 2018 1 1 1 1.0 Markey MA 2020 1 1 1 1.0 Warren MA 2018 1 1 1 1.0 Leahy VT 2022 1 1 1 1.0 Cardin MD 2018 1 1 1 1.0 Sanders VT 2018 1 1 1 1.0 Schumer NY 2022 1 1 1 1.0 Which Democrats might support Trump? *Campbell’s position is pending results of Dec. 10 runoff in Louisiana Sources: OnTheIssues.org, Daniel Nichanian, David Wasserman

Heitkamp, Manchin and Donnelly (and Campbell, if he wins the Louisiana runoff) are probably the biggest defection risks for Democrats, not being all that far apart from Trump on the issues and coming from extremely red states that are voting again in 2018. Sen. Jon Tester of Montana and Sen. Claire McCaskill of Missouri also face tough re-election bids, but they’re further removed from Trump on the issues and have a track record of standing with their party on major votes.

But if the bad news for Democrats is that three to six of their votes could potentially be in play depending on the issue, the good news is that the rest of their senators are likely to be pretty unified. Because Democrats lost so many Senate races in swing states in 2014 and 2016, most of their remaining senators come from reliably blue states, and also have reliably liberal voting records. Some 43 Democrats have a Trump support score of 2.0 or lower, which is two more than they need to sustain a filibuster.

In short, the Senate — after a relatively dormant period during President Obama’s second term — will be the center of attention again once Trump enters office. There will potentially be a fair amount of party crossover, with Democratic senators supporting Trump and Republican ones opposing him, sometimes on the same vote. Relatively minor differences in Trump’s popularity could make a big difference in whether his agenda is passed or stymied, as these senators calculate the impact of their vote in 2018 or 2020. And special elections — such as the one that brought Massachusetts’ Scott Brown into the Senate in 2010 — could make a big difference. But with a large GOP majority in the House of Representatives, the Senate will be the closest thing to a check on Trump’s power until voters go to the polls again.

CORRECTION (Nov. 21, 10:41 a.m.): A previous version of this article misidentified one of the two Republican-controlled Senate seats Democrats won in 2016. It was New Hampshire, not Nevada.

CORRECTION (Nov. 21. 12:24 p.m.): A previous version of this article included tables that incorrectly calculated each senator’s Trump support score. The calculation did not properly weight the score based on the year in which the senator is up for re-election, as the article stated. The tables have been corrected to match the calculation method described in the article, and the relative ranking of some senators has shifted slightly as a result. In addition, a table mistakenly listed Senator-elect Kamala Harris of California and Sen. Patty Murray of Washington as being up for re-election in 2020. They will next be on the ballot in 2022.