Democrats are falling in line. Republicans are falling apart.
The most consequential night of voting so far in the presidential campaign crystallized, in jarring and powerful fashion, the remarkably divergent fortunes of the two major parties vying for the White House.
The steady and seemingly inexorable unification of the Democratic Party behind Hillary Clinton stands in striking contrast with the rancorous and widening schisms within the Republican Party over the dominance of Donald Trump, who swept contests from the Northeast to the Deep South on Tuesday night.
Now, as the parties gaze ahead to the fall, they are awakening to the advantages of consensus and the perils of chaos.
"If the Republican Party were an airplane, and you were looking out a passenger window you would see surface pieces peeling off and wonder if one of the wings or engines was next," said Tim Pawlenty, a former governor of Minnesota and a Republican candidate for president in 2012.
Even as he rolled up commanding victories in at least six states Tuesday, Trump confronted a loud and persistent refusal to rally around him, as leading figures in his own party denounced his slow disavowal of white supremacists, elected officials boldly discouraged constituents from backing him and lifelong Republicans declared that they will boycott the election if he is their nominee.
"I could not in good conscience vote for Trump under any circumstance," said Blake Lichty, 33, a Republican who worked in the George W. Bush administration and now lives outside Atlanta.
"If this becomes the Trump Party," he added, "we're going to lose a lot of people."
Not since the rupture of 1964, when conservatives seized power from their moderate rivals and nominated Barry M. Goldwater of Arizona has a major party faced such a crisis of identity.
"History is repeating itself," said historian Richard Norton Smith. "The party changed then as permanently and profoundly as can be in politics, effectively becoming two parties."
Even as Trump's performance Tuesday illustrated his strength, Sen. Ted Cruz's success in Texas and Oklahoma underscored the broader Republican dilemma: there is no consensus among Republicans about who could be Trump's most formidable opponent, and there is probably not enough time for one to emerge.
The cultural and ideological fissures now opening in the Republican party could take a generation to patch, according to Republican leaders, historians and strategists — and many in the party are convinced that Trump will guarantee Democrats another four years in the White House.
"Nominating Donald Trump would be the best gift the Republican Party could give to Hillary Clinton," Bobby Jindal, the former Louisiana governor, said in an interview Tuesday.
Democrats are now poised to exploit a fortuitous intersection of forces: an improving economy with low unemployment; a Democratic president with a nearly 50 percent approval rating; a Supreme Court battle in which Republicans are energizing liberal voters with vows of obstruction; and now, what is likely to be a relatively smooth nomination process that will give her a chance to bring together the disparate strands of the party.
Of course, Clinton, should she prevail in the primary, has plenty of repair work left to do: wooing the thousands of liberal supporters whose feelings of alienation with the Democratic establishment drew them to Sen. Bernie Sanders of Vermont. She began that effort Tuesday night, in a victory speech that focused heavily on the Sanders campaign theme of economic justice. And there is little indication, so far, that these voters will spurn Clinton for a Republican.
And Clinton needs to navigate a series of potentially damning investigations into her use of a private email server that have raised enduring questions about her judgment and management. Those inquiries have introduced a level of unpredictability that her campaign can do little to control.
But officials in both parties acknowledge that Democrats are now better positioned to capture the presidency in November.
"The Democrats are having a loud squabble, but the party is broadly unified behind certain themes," said David Axelrod, the Democratic strategist. "The Republicans are engaged in a full-out civil war, fundamentally riven by mistrust, and it is very hard to see how they put the pieces back together once this fight is done."
With every nasty turn of the Republican nominating contest, Clinton's position seems to strengthen. Day by day, the anti-Trump forces are marshaling, vowing to drag the primary process out until the convention in July.
In an extraordinary show of defiance toward a potential presidential nominee, Sen. Ben Sasse, R-Neb., wrote an open letter to Trump supporters explaining why he could not support the real estate mogul should he become the party's nominee.
"I sincerely hope we select one of the other GOP candidates," Sasse wrote, pledging not to vote for Clinton. "But if Donald Trump ends up as the nominee, conservatives will need to find a third option."
In the past 48 hours, Rep. Scott Rigell of Virginia appealed to fellow Republicans in his state to reject Trump, calling him "a bully unworthy of our nomination," and Gov. Susana Martinez of New Mexico would not commit to supporting him if he won the nomination.
House Speaker Paul D. Ryan of Wisconsin, the party's vice presidential nominee four years ago, took the unusual step of scolding the Republican front-runner from the halls of the Capitol building for failing to reject the support of David Duke.
"If a person wants to be the nominee of the Republican Party," Ryan said, "there can be no evasion and no games. They must reject any group or cause that is built on bigotry."
In a discussion with little modern precedent, several high-profile Republicans are expressing uncertainty about how aggressively they would support Trump as the nominee, suggesting they might need to lose the campaign to save the party.
"President Trump, which I don't believe is possible, would be an unmitigated disaster and would set the party back decades," said Mike Murphy, a longtime Republican strategist who oversaw the super PAC supporting Jeb Bush this year. "It's like a computer designed him to lose elections for us. Who does he offend? College-educated white women and Latinos, the groups we need to win."
But any move to deny Trump the nomination risks further provoking the angry movement that he has ignited.
Heather Cox Richardson, a Boston College professor and author of a new history of the Republican Party, predicts a violent rupture that cleaves the party in two: a hard-line conservatism, as embodied by Pat Buchanan, Newt Gingrich and Trump, and an old-fashioned strain of moderate Republicanism that recalls Teddy Roosevelt, Dwight Eisenhower and Nelson Rockefeller.
"It is going to be really ugly," she said.
For now, the revulsion for Trump could produce a nightmare scenario for Republicans on Election Day: abandonment by rank-and-file voters who, like a growing number of party leaders, cannot stomach the concept of the mogul as their standard-bearer.
"I think it's a sad day for the Republican Party," said David Phillips, 72, an executive recruiter and longtime Republican from Avon, Connecticut, who called Trump "a tremendous divider."
"If he were the nominee," he said, reluctantly, "I would probably vote for Hillary."
Flegenheimer, and Thomas Kaplan contributed reporting.