By seizing the Republican presidential nomination for Donald J. Trumpon Tuesday night, he and his millions of supporters completed what had seemed unimaginable: a hostile takeover of one of America’s two major political parties.

Just as stunning was how quickly the host tried to reject them. The party’s two living former presidents spurned Mr. Trump, a number of sitting governors and senators expressed opposition or ambivalence toward him, and he drew a forceful rebuke from the single most powerful and popular rival left on the Republican landscape: the House speaker, Paul D. Ryan.

Rarely if ever has a party seemed to come apart so visibly. Rarely, too, has the nation been so on edge about its politics.

Many Americans still cannot believe that the bombastic Mr. Trump, best known as a reality television star, will be on the ballot in November. Plenty are also anxious about what he would do in office.

But for leading Republicans, the dismay is deeper and darker. They fear their party is on the cusp of an epochal split — a historic cleaving between the familiar form of conservatism forged in the 1960s and popularized in the 1980s and a rekindled, atavistic nationalism, with roots as old as the republic, that has not flared up so intensely since the original America First movement before Pearl Harbor.

Some even point to France and other European countries, where far-right parties like the National Front have gained power because of the sort of resentments that are frequently given voice at rallies for Mr. Trump.

Yet if keeping the peace means embracing Mr. Trump and his most divisive ideas and utterances, a growing number are loath to do it.

The ties between Republican elites — elected officials, donors and Washington insiders — and voters have actually been fraying for years. Traditional power brokers long preached limited-government conservatism and wanted to pursue an immigration overhaul, entitlement cuts, free trade and a hawkish foreign policy, and nominees like John McCain and Mitt Romney largely embraced that agenda. Republican leaders also vilified President Obama and Democrats, stoking anger with rank-and-file conservatives.

Many Republican voters trudged along with those earlier nominees, but never became truly animated until Mr. Trump offered them his brand of angry populism: a blend of protectionism at home and a smaller American footprint abroad. And he was able to exploit their resentments and frustrations because those same Republican leaders had been nurturing those feelings for years with attacks on Mr. Obama, Democrats, illegal immigrants and others.

Mr. Trump, with his steadfast promises to deport immigrants who are in the country illegally and to build a wall with Mexico, may have done irreversible damage to his general election prospects. But he quickly earned the trust that so many of those voters had lost in other fixtures of America — not just in its leaders, but in institutions like Congress, the Federal Reserve and the big-money campaign finance system that Mr. Trump has repudiated, as well as in corporations, the Roman Catholic Church and the news media.

And he has amplified his independent, outsider message in real time, using social media and cable news interviews — and his own celebrity and highly attuned ear for what resonates — to rally voters to his side, using communication strategies similar to those deployed in the Arab Spring uprising or in the attempts by liberals and students to foment a similar revolution in Iran.

“Trump leveraged a perfect storm,” said Steve Case, the founder of AOL, in an email message. “A combo of social media (big following), brand (celebrity figure), creativity (pithy tweets), speed/timeliness (dominating news cycles).”

Mr. Trump is an unlikely spokesman for the grievances of financially struggling, alienated Americans: a high-living Manhattan billionaire who erects skyscrapers for the wealthy and can easily get politicians on the phone. But as a shrewd business tactician, he understood theRepublican Party’s customers better than its leaders did and sensed that his brand of populist, pugilistic, anti-establishment politics would meet their needs.

After seething at Washington for so long, hundreds or thousands of miles from the capital, many of these voters now see Mr. Trump as a kind of savior. Even if he does not detail his policies, even if his language strikes them as harsh sometimes, his supporters thrill more to his plain-spoken slogans like “Make America Great Again” than to what they see as the cautious and poll-tested policy speeches of Mr. Ryan and other Washington Republicans.

“I love the death out of Paul Ryan, but honestly, I’m going to vote for Trump anyways,” said David Myers, 49, who attended a campaign rally for Mr. Trump in Charleston, W.Va., on Thursday night. “Because Paul Ryan, and I love him to death, but he’s one of those career politicians.”

Mr. Trump now feels so empowered that he does not think he needs the political support of the party establishment to defeat the likely Democratic nominee, Hillary Clinton. He is confident that his appeal will be broad and deep enough among voters of all stripes that he could win battleground states like Michigan, Ohio, and Pennsylvania without the support of leaders like Mr. Ryan, Mr. Trump said in an interview on Saturday.

Although he plans to meet with Mr. Ryan and House Republican leaders on Thursday, Mr. Trump said he would not materially change his policies or style to win their endorsements. “Everything is subject to negotiation, but I can’t and won’t be changing much, because the voters support me because of what I’m saying and how I’m saying it,” Mr. Trump said. “The establishment didn’t do anything to make me the nominee, so its support won’t really make much difference in me winning in November.” (Mr. Trump will, though, be somewhat dependent on the party’s fund-raising muscle since he has indicated he will not fully self-finance his general election campaign.)

One reason Mr. Trump takes a skeptical view of establishment support is that he does not believe much in the power of the Republican elite. He is the party’s presumptive nominee, after all, because the political forces that once might have halted his rise have been enfeebled. Leaders such as Mr. Romney warned in the direst terms that Mr. Trump’s nomination would stain the party and lead it to ruin. Venerable media outlets on the right, like National Review, sought to reprise their role as arbiters of who is fit to carry the banner of conservatism. Their pleas fell on deaf ears.

In doing so, he seemed to grasp that a new twist on direct democracy was in the offing: that disaffected voters who tune out the traditional modes of political communication might be reachable through their smartphones, and Twitter messages or Reddits might be more relevant to those voters than the findings of a more scientific poll.

On the left, too, Senator Bernie Sanders has built his own movement with millions of voters, and $210 million in fund-raising, by using online tools as simple as email to seek support. Yet Mr. Trump’s celebrity has been an enormous asset with voters who feel gratified and inspired that he would lavish them with attention and bluntly express some of the ideas and attitudes they share.

For 12 consecutive years, polls have indicated that Americans believe the country is on the wrong track, and Republicans have been especially vulnerable to a political campaign like Mr. Trump’s that seeks to channel voter anger. In every state where the question was asked in exit polls during the primary season, 50 percent or more of Republicans said they felt betrayed by their leaders.

The adhesive that once held Republicans together — a shared commitment to a strong national defense and limited government — was weakened by the demise of the Soviet Union and the end of the Cold War. But internal divisions were papered over when new, unifying threats emerged after the attacks on Sept. 11, 2001.

It was not until near the end of President George W. Bush’s second term that those fissures broke open again, first with Mr. Bush’s attempt at an immigration overhaul, including a path to citizenship for illegal immigrants, and then after the financial rescue of big banks from the 2008 financial collapse.

Alongside the turbulent economy were signs of something more profound plaguing blue-collar white communities, which have increasingly become core Republican constituencies: an increase in children born to single parents, higher rates of addiction and suicide, and shortened average life spans.

“The economic deprivation of the last 30 years for working-class whites, combined with growing social isolation, was really dry tinder,” said Robert D. Putnam, the Harvard political scientist who wrote “Bowling Alone.” And Mr. Trump, Mr. Putnam contended, “lit a spark.”

“He constructed a series of scapegoats that these folks would find plausible,” said Mr. Putnam, citing Mr. Trump’s attacks on Muslims and immigrants. “He was willing to say things that might have always been popular, but you couldn’t say it.”

With Mr. Trump now saying it loudly and clearly, many Americans feel deeply unsettled by the nation’s politics. Not since Mr. Bush invaded Iraq have so many liberals been murmuring about moving to other countries. And many Republican officials and donors just hope to get through the election with their party intact.

“The party has never been more out of touch with our voters,” Vin Weber, a former Minnesota congressman, said of the two factions, acknowledging that Republicans could splinter completely after this election. “I don’t know how you reconcile a lot of them.”

Mr. Weber expressed hope that Mr. Trump and Mr. Ryan would find some common ground. But few in the party now deny that the threat of an enduring split is real.

“I think there’s a pretty clear Trump wing of the party coming to life,” said Barry Wynn, a prominent fund-raiser who supported Jeb Bush for president and has not yet fallen in behind Mr. Trump. “But I have to think that four or eight years from now, the Trump wing will be a little more traditional, a little less hard-edged, and will be blended into the party just like the evangelical Pat Robertson voters were after the 1988 election.”

“At least,” he added, “I hope that’s what’ll happen.”

Republican Party Unravels Over Donald Trump’s Takeover