With no other primary candidates to oppose him and the Republican Party rapidly falling into line, Donald Trump is soaring in national polls, while Hillary Clinton’s double-digit lead has evaporated in the space of a month. On Sunday, the presumptive G.O.P. nominee passed Clinton for the first time in a weekly average of head-to-head polls, edging the former secretary of state by 0.2 points—a virtual dead heat that is the result of an undeniable and dramatic shift in recent days. Of the last five polls released since Wednesday, three show Trump in the lead: Fox News has the billionaire winning by 45-42; Rasmussen shows him up 42-37; and the latest ABC News/Washington Post survey leans toward Trump by a 46-44 margin.

A “Trump Bump” among Republicans is not surprising. Despite pockets of resistance among many conservative intellectuals, ashen-faced party leaders are predictably swallowing their pride and embracing the front-runner—even if, like Lindsey Graham, they are too embarrassed to endorse Trump in public. House Speaker Paul Ryan may still be sputtering, but the rank-and-file are already looking ahead to the general election.

Clinton, meanwhile, is growing more unpopular by the minute as she faces an unexpected eleventh-hour rally by Sanders. Last month, in the wake of a Super Tuesday shellacking, the Vermont senator indicated he would be dialing back his attacks on the Democratic front-runner to run an “issue-oriented” campaign. But the cease-fire came to an abrupt end last weekend, when Sanders’s supporters revolted at the Nevada state convention, reportedly throwing chairs and hurling insults at Democratic officials they accused of tilting the scales for Hillary. Sanders barely apologized, inveighing against the D.N.C. and terrifying party leaders with his newly belligerent tone. Despite the fact that it is practically impossible for the Vermont senator to catch up Clinton, Democrats are now grappling with the prospect of an ugly, drawn-out fight that could roil their own convention in July.

Fighting a two front war, with Sanders to her left and Trump to her right, Clinton’s unfavorables have risen steadily, reaching a new high in the last week. According to Gallup polling, Clinton is now as disliked by Democrats (29 percent unfavorable) and Trump is unpopular with Republicans (30 percent unfavorable). “If you look at the favorability ratings of Donald Trump and Hillary Clinton, both of them have very, very high unfavorables,” Sanders said in an interview Sunday, characterizing the choice as Americans being forced to pick between the lesser of two evils. “If you look at virtually all of the polls done in the last six, seven weeks, in every one of them, nationally polls and statewide polls, we defeat Trump by larger margins—in some cases, significantly larger margins—than does Secretary Clinton,” he continued.

Clinton is counting on the threat of Donald Trump to ultimately unify Democratic voters behind her and energize supporters to get out to the polls. But the longer Sanders stays in the race, the harder it becomes for her to pivot to the general election and focus her attacks on the Republican. “I think it’s certain that Clinton would benefit from putting the intra-party divisiveness behind her in the same way that Trump has benefited from that on his side,” Democratic pollster Geoff Garin, who advises a pro-Clinton super-PAC, told Politico. “Polling in 2008 reflected a unification among Democrats after the last primary as a result of Hillary expressing her support for Obama and encouraging her own supporters to also get behind him.”

Sanders is expected to do the same, when his presumed loss to Clinton becomes official this summer. But holding his endorsement—not to mention his supporters and massive voter data bank—hostage until he exacts concessions from the Democratic Party platform in Philadelphia practically ensures that Clinton’s position vis-à-vis Trump will have been weakened. Whether she can make up for lost ground then, one week after Republicans will have just coronated their candidate in what is sure to be a dazzling convention spectacle in Cleveland, could be a more serious challenge than navel-gazing leftists assume.