Since Iowa is looking like a 2020 swing state, Mr. Trump will probably hold many rallies there, trying to drive up support for himself and for Ms. Ernst. But the Bevin example shows that unless Ms. Ernst gets her numbers up on her own and relatively soon, she could be in trouble. She may be able to save herself; Mr. Trump, it appears, cannot.

The same is true, albeit to a less dire degree, for Cory Gardner in Colorado. He is underwater, with a 36 percent approval rating against a 39 percent disapproval rating. In addition to those uninspiring numbers, he will almost certainly face stiff competition, most likely drawing a strong opponent in the former governor John Hickenlooper. That it’s a presidential year will also not play to his favor — Colorado hasn’t voted for the Republican nominee for president since 2004.

This is also true for Susan Collins — only Mr. McConnell possesses a more troublesome disapproval number. It appears that the confirmation of Brett Kavanaugh to the Supreme Court was a real, and lasting, problem for her. And as with Iowa in 2018, in the shadow of Mr. Trump, the state shifted away from Republicans, tossing Representative Bruce Poliquin in favor of a Democrat and flipping the governorship from red to blue.

Ms. Collins didn’t vote for Trump; she wrote in Paul Ryan. That suggests there’s zero chance she’ll want Mr. Trump rallying for her, but also that there’s zero chance that the president will be minded to rally his base to save her. That’s just as well. She has survived before in extremely tough years for Republicans — namely 2008 — and Kentucky showed he just isn’t the closer he makes out to be.

All of this is occurring, of course, against the background — and moving target — of impeachment and the 2020 Democratic nominating contest. Mr. Trump’s approval numbers are better than those of Senators McConnell, Ernst and Gardner, but remarkably, he’s never hit — or even really approached — 50 percent. About 48 percent, and more respondents than not, support impeaching and removing him from office at this point.

If his approval numbers inch down and support for impeachment and removal grows, the outlook for these Republican senators — and others — might become even more bleak. And they may start considering options to buy a reprieve that would be helpful to their re-election chances but harmful to the president: If they are so inclined, they might give more private thought of voting to remove Mr. Trump from office. Or they might be more open to a symbolic vote to convict and remove, knowing not enough other Republican senators would follow. Either one of those scenarios hurts Mr. Trump’s political fortunes by lending legitimacy to the impeachment process.

The lesson to be taken by the Republican Party from the Bevin debacle is that senators and others need to work on their own images and reputations to seal the deal with voters — and apart from Mr. Trump and his popularity with his base, which might not be transferable. He couldn’t elevate Mr. Bevin, and his own data as well as public polling indicated that he should have. He’s not going to be the savior of Senators McConnell, Ernst, Gardner or Collins, either — and all of them should be planning accordingly.