In the House, Democrats would need to net 30 seats to win back a majority, but in a wave election, underdogs often prevail. The Cook Political Report rates 31 GOP-held seats as competitive (either as toss-ups or lean Republican). Of those 31 districts, 23 are based in urban or suburban areas where Trump’s brand of populism is unlikely to be a selling point.
There are two main arguments that Republicans are relying on to persuade themselves that Trump’s nomination wouldn’t be catastrophic. One is that even presidential candidates who lose in landslides don’t usually bring their party down with them. Democrats picked up Senate seats in George McGovern’s and Walter Mondale’s embarrassing elections. And Republicans only lost one Senate seat in Barry Goldwater’s epic 1964 defeat. But the party of all three losing candidates suffered significant House losses, and it’s hard to see in today’s polarized political environment how Trump’s divisive candidacy wouldn’t ripple down-ballot.
Second, some GOP strategists believe that Trump’s ability to attract new voters could give a populist boost to established senators. In Ohio, as the theory goes, the establishment-friendly Portman has already locked down college-educated Republicans and could benefit from Trump’s reorienting of blue-collar voters in the GOP’s corner. But this theory ignores Trump’s damage to the GOP brand, which could make college-educated swing voters less comfortable voting for Portman or any Republican.