House Speaker Paul Ryan downplayed Trump's recent attacks on Special Counsel Robert Mueller.Joshua Roberts / Reuters

Every time Donald Trump breaks a window, congressional Republicans obediently sweep up the glass.

That’s become one of the most predictable patterns of his turbulent presidency—and a defining dynamic of the approaching midterm elections. Each time they overtly defend his behavior, or implicitly excuse him by failing to object, they bind themselves to him more tightly.

It happened again last weekend when Trump fired off a volley of tweets that, for the first time, attacked Special Counsel Robert Mueller by name. A handful of GOP senators responded with warnings against dismissing Mueller. More congressional Republicans said nothing. Party leaders, such as House Speaker Paul Ryan, tried to downplay the attacks by insisting that Trump would not act on them and fire Mueller, who is investigating Russian interference in the 2016 presidential election. Most important, and regardless of their rhetorical posture, Republicans almost universally locked arms to reject legislative action to protect the special counsel.

That reaction reflected a hardening pattern. Whatever the provocation—reported payoffs to a porn star, a chaotic security-clearance process in the White House, the public belittling of Attorney General Jeff Sessions—congressional Republicans have found ways to excuse or simply ignore behavior that would have launched a thousand subpoenas under a Democratic president.

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.

One group has emerged as especially alienated from the president: college-educated white women. The group ordinarily leans Democrat, but only slightly: Since 1992, Democrats have never carried more than 52 percent of their votes in House elections, and Hillary Clinton won 51 percent of them in 2016. However, this week’s NBC/WSJ poll found that 63 percent of them now disapprove of Trump and 62 percent intend to vote Democratic in November.

By itself, such a sharp move among college-educated white women could doom many Republicans clinging to their seats in white-collar districts in major metro areas. Reinforcing the danger is the president’s decline since 2016 among blue-collar white women and college-educated white men. House Democrats only won about 35 percent of each group in both the 2010 and 2014 midterms. This week, both groups split about evenly in their congressional preferences in the NBC/WSJ survey.

Even if Republicans energize their base enough to avoid the worst in November, polls are clarifying the long-term risks of welding themselves to Trump. With Millennials poised to pass the baby boom in 2018 as the largest generation of eligible voters, the NBC/WSJ survey found that two-thirds of Millennial women disapprove of Trump and nearly three-fourths intend to vote Democratic for Congress. (Democrats had a much narrower six-point lead among Millennial men.)

That gaping advantage underscores separate findings this week from the Pew Research Center showing that the Democratic edge over Republicans in party identification among Millennials has significantly widened since 2014 because of an exploding gap among young women. Republicans under Trump are trading younger voters for older, white-collar workers for blue collar, and booming metros for less dynamic small-town and rural areas.

For congressional Republicans, the choice to tie themselves to Trump now looks irreversible. The question remains whether they have fashioned a lifeline or a noose.

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