Looking ahead to November, the presidential race could decide more than just who the next president is. It could also determine which party controls the Senate or House or even a state’s legislature. And that’s because of something political science calls the “coattail effect,” which is when the popularity of a candidate at the top of the ticket — we’re most interested in presidential coattails — boosts the fortunes of candidates from the same party lower down on the ballot.
Political science has long tried to measure just how big of an effect this is, and most research does suggest that down-ballot candidates gain at least a few votes thanks to the performance of the candidate at the top of the ticket, but estimates vary as to the actual size of the coattail effect. For instance, in House races, where we have the most data, some studies have found anywhere from a 0.2-point to 0.5-point bump for a House candidate for every 1-point increase the presidential candidate experiences in the vote share. That may not sound like much, but if a candidate in a competitive House seat has attracted 48 percent of the vote by herself, a strong performance by the same party’s presidential candidate could boost her vote share just enough that she clears the 50 percent mark and wins.
And as the chart below shows, the margins in presidential and House races do tend to move together. This alone doesn’t prove the existence of a coattail effect, but it does show how closely these votes are connected. For instance, in 2004, President George W. Bush won reelection by 2.5 points while the GOP won the national House vote by 2.6 points. Of course, it’s not always so symmetrical — in 1996, President Bill Clinton won reelection by 8.5 points, but Democrats won the House vote by only 0.3 points. Overall, though, from 1992 to 2016, we found a strong correlation (0.655) between the national margins for presidential and House races.1
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https://fivethirtyeight.com/features/how-much-trump-could-help-or-harm-down-ballot-republicans-in-november/
