With all of this, the GOP leadership is sending an unmistakable signal to voters antagonistic or ambivalent about Trump: So long as Republicans hold the congressional majority, they will not act to meaningfully constrain, or even oversee, the president. That choice represents an epic gamble for November—and beyond.

All the elections held since 2016 have signaled that Republicans are facing elevated turnout among Democratic partisans eager to hobble Trump. The refusal to challenge Trump—particularly as he rages against the Russia investigation—enhances that risk for Republicans. Their approach threatens to persuade less partisan voters that they need a Democratic House (and perhaps Senate) to impose any limits on a president who daily redefines the words “mercurial,” “belligerent,” and “volatile.”

“When one party controls the House, the Senate, the presidency, the losses get exaggerated [in a midterm election], because independent voters tend to put a check on the president rather than giving him a blank check,” said Tom Davis, a former Republican representative from Virginia and a former chair of the party’s House campaign committee. “I think that [sentiment] is very intense right now given the disruption we’re seeing in the White House.”

Attitudes about Mueller’s inquiry quantify the impulse to constrain Trump. In a February CNN poll, 61 percent of Americans said the Russia matter was a serious issue that needed to be fully investigated; just 34 percent said the probe was mainly an effort to discredit Trump. Of the latter group, about three-fourths said they intended to vote Republican for Congress—but nearly four-fifths of the (larger) concerned group said they will vote Democratic, according to figures provided by CNN Polling Director Jennifer Agiesta.

Some GOP strategists believe the imperative of energizing the GOP base—which preponderantly supports Trump—justifies the risk of alienating less partisan voters inclined to restrain him. And in some Republican-leaning places, that calculation may compute. But in almost all swing House districts, “you can’t get to 50 percent [of the vote] with just base voters,” noted Meredith Kelly, the communications director for the Democratic Congressional Campaign Committee. That threshold requires “crossover support and independents”—the sort of voters that may prefer some constraints on Trump, even if they don’t entirely reject his direction.

As American politics has grown more tribal since the 1990s, attitudes toward the president have become a decisive factor in congressional elections. In each midterm since 1994, 82 percent to 86 percent of the voters who disapproved of the incumbent president voted against his party’s House candidates, exit polls found.

That effect may be even more intense under Trump because such a high proportion of those who disapprove of him do so strongly: An Election Day poll in last week’s Pennsylvania special election, for instance, found that fully 93 percent of Trump disapprovers backed Democrat Conor Lamb, the victor. In this week’s NBC/Wall Street Journal poll, 87 percent of Trump disapprovers said they intend to vote Democratic for Congress.