Donald Trump’s approval rating has been hovering at around 37 percent. He’s faced with a metastasizing investigation run by a dogged special counsel. He has no legislative accomplishments to speak of. Increasingly, it appears that 2018 will be a referendum on his presidency, and a disaster for Republicans. And yet, still, the vulnerable G.O.P. won’t break with him. Why? “Politics in Washington is driven by fear, and it’s fear of not getting elected,” Andrew Smith, a political-science professor and the director of the Survey Center at the University of New Hampshire, said in an interview. “You’re not gonna see a whole lot of profiles in courage coming up . . . regardless of what happens.”

Vulnerable Republicans are faced with an exquisitely painful calculation, a Trump trap that increasingly looks like a lose-lose. Leave Trump, and be dragged down by his base. Keep him close, and potentially be crushed by independents and highly motivated Democrats. Despite Trump’s plummeting overall approval numbers amidst the chaos of the investigation and policy failures, his base has been much steadier. “They are Trump supporters first and foremost, issues [are] secondary,” Doug Heye, former communications director for the Republican National Committee, said. “They like that he’s making the right enemies . . . they like that Trump is willing to pick the fights that they like.”

Among this slice of the electorate, Trump is still more popular than the generic Republican—along with many of the G.O.P. candidates up for re-election in 2018—in most districts he carried last fall. “Are you going to proactively anger those voters? I don’t think so,” Heye said. A top Republican strategist, who requested anonymity to speak candidly, refined the arithmetic: “Everybody does better being closer to him than trying to be farther away, because the closer they are to him, the better he will do because we’re all unified,” he said. “If Trump goes up two points, you go up two points. If he goes down and you try and break with him, you just lose some of the Republicans that actually like him and you really don't pick anybody up on this side . . . For every vote you lose on a Republican right now, you don’t gain it with an independent.”

The dispiriting possibility is that, while sticking with Trump could very well be the better strategy, it might still be a losing one. “Nobody has a sense of what to do, because they’ve inherited all the downsides of Trump and none of the up. They don’t have the 100 percent name I.D., the celebrity status, all that other shit,” Republican strategist and well-known anti-Trumper Rick Wilson said. “They have a much higher hill to climb and a much tougher hill to climb, in their races. The Republican voters that love Trump don’t necessarily love them. In fact, people who are pure Trump voters, they hate Republicans just about as much as they hate the Democrats. They hate the swamp, they hate the establishment.”

Trump’s base has eroded modestly. His strongly-approve number has declined from a peak of 33 percent in February to 25 percent in a recent poll. But for Republicans, that’s still a terrifying number.

The Republican Party’s reluctance to distance itself from Trump should not be confused with contentment with the president. “The general tenor and stability of the administration has a lot of members of Congress shaking their heads. What they’re saying privately is not what they’re saying publicly. They’ve been very critical,” Heye said. “For a lot of members that I’ve talked to, it’s not a Trump fatigue but a larger Trump exasperation that there doesn’t seem to be a lot of consistency from the White House, that there’s not the direction, that they still aren’t putting people in key positions that don’t need Senate confirmation.”