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How Republicans Made Gains In The Senate

Midterms 2018: FiveThirtyEight reacts to election results

On a night filled with mixed results for both parties, partisanship mostly prevailed in the Senate, to the benefit of Republicans. According to ABC News’s projections, they flipped three seats and lost one, with three more races (Montana, Florida, Arizona) not yet called, though Republicans lead in all three. If those three straggler races are confirmed for the GOP, the party’s majority will expand from 51 seats to 54, with a runoff election in Mississippi still to come. This result was was one of the more favorable Republican outcomes in our forecast of how Election Night would go in the Senate.

Depending on how things shake out in the remaining races, it’s possible that 30 of 35 Senate races (86 percent) will have voted for the same party that won the state at the presidential level in 2016.1 Compared to past elections, that rate would be very high for a midterm.

En route to its gains in the Senate, the GOP defeated three Democratic incumbents: Republican Mike Braun beat Indiana Sen. Joe Donnelly by 10 points, Republican Josh Hawley defeated Missouri Sen. Claire McCaskill by 6 points, and Republican Rep. Kevin Cramer beat North Dakota Sen. Heidi Heitkamp by 10. All three seats were in states that President Trump carried in 2016, and they were going to be tough for Democrats to hold given the states’ partisan lean toward the GOP. Add in Florida, which looks likely to be called against incumbent Democratic Sen. Bill Nelson,2 and a Republican takedown of that quartet would be historically notable. Four Senate incumbent defeats for the non-presidential party in a midterm would be the highest number since eight Republican incumbents lost in 1934.

It’s another reminder of how bad the Senate map was for Democrats. They had to defend 10 seats in states that Trump won, and lost at least three — and maybe as many as five — of them.

Further evidence of partisanship winning out can also be found in Arizona and Nevada. Trump won in Arizona in 2016, and the GOP is narrowly holding onto a lead in the race for an open seat there. Republican Rep. Martha McSally is leading Democratic Rep. Kyrsten Sinema 49 percent to 48 percent in the race to become Arizona’s first woman senator. In Nevada, where Hillary Clinton won in 2016, Democrats got their one gain of the night when Rep. Jacky Rosen defeated Republican Sen. Dean Heller by about 4 points.

At least one Democratic incumbent survived despite the strong GOP lean of his state. Joe Manchin of West Virginia won his race by just 4 points, but he overcame the state’s more than 30-point GOP lean to do it. Notably, Manchin was the one Democrat to vote for Brett Kavanuagh’s confirmation to become a justice on the U.S. Supreme Court. At press time, the fate of Democratic Sen. Jon Tester is unknown, as his race against Republican Matt Rosendale is extremely close (Rosendale is leading by a percentage point with 94 percent of precincts reporting).

Some other results merit mentioning. Republican Sen. Ted Cruz defeated Democratic Rep. Beto O’Rourke by less than 3 points in the Texas Senate race, a narrow result that might set O’Rourke up for another future electoral bid — most obviously a presidential run in 2020. O’Rourke can credibly make the case that his campaign boosted turnout and helped Democrats win two House seats. Another reach seat for Democrats was Tennessee, where Republican Rep. Marsha Blackburn easily dispatched Democrat Phil Bredesen by 11 points. And finally, Democratic incumbents comfortably won re-election in Rust Belt states such as Michigan, Ohio, Pennsylvania, and Wisconsin that Trump carried in 2016.


  1. Maine and Vermont’s independent senators caucus with the Democrats, so I am counting their re-elections in the Democratic column for this exercise.

  2. Because Florida law requires a recount for races with a margin under 0.5 points, it may be some time before the election is officially called. However, it is unlikely that a recount would alter the outcome.

Geoffrey Skelley is a senior elections analyst at FiveThirtyEight.