How much Trump could help (or harm) down-ballot Republicans in November

Experts argue that where we really see the coattail effect is in turnout among a party’s base. “The way [the top of the ticket is] going to benefit the down-ticket candidates the most is by getting members of that party to show up to vote,” said Meredith. This is especially true in our polarized era that has high rates of straight-ticket voting. “For every 10 partisans to get out to vote for a presidential candidate, you’re probably getting eight or nine of them voting for the [same party’s] House candidate,” added Campbell.

But one reason why the coattail effect isn’t more pronounced is a countervailing force at work known as “balancing,” which is when someone votes against the party they think will win the presidency in down-ballot races. Looking at elections from 1948 to 2012, political scientist Robert Erikson of Columbia University found that if a presidential candidate was heavily favored to win, highly engaged, moderate voters often cast House votes for the other party — an “ideological hedge” of sorts. It’s true that only a relatively small number of voters do this, and split-ticket voting for president and Congress has generally declined in recent years, but enough voters still do this that it can make a difference in House races.