At the core of Donald Trump's political success this year are the grievances of a sizable and now vocal block of disaffected voters, many of them white and working-class, and a Republican Party that has sought and benefited from their support while giving them almost nothing tangible in return.
The New York businessman's position as the front-runner for the Republican presidential nomination has plunged the party into a contentious debate and raised some of the most troubling questions about its future since the Watergate scandal in 1974 or Barry Goldwater's landslide defeat a decade earlier.
Campaigning on Friday, Sen. Marco Rubio (Florida), who is seeking to deny Trump the nomination, put the threat in apocalyptic terms. If Trump becomes the nominee, he said, "He will split the Republican Party and it will be the end of the modern conservative movement."
Trump and so-called Trumpism represent an amalgam of long-festering economic, cultural and racial dissatisfaction among a swath of left-out Americans who do not fit easily into the ideological pigeonholes of red and blue, right and left.
James W. Ceaser, a professor in the politics department at the University of Virginia, describes the eruption behind Trump as less an "ism" and more "a mood" that has been at a near-boil for some time. But why has it hit with such force in this election? "They have a leader who can articulate it," Ceaser said.
The sight of establishment Republicans recoiling at Trump strikes some analysts, particularly on the left, as ironic. These GOP critics see Trump's appeal as the logical result of decades of efforts by the GOP to discredit government and more recently of the party leadership's passive acceptance of virulent and in some cases racially tinged opposition to President Obama. Having sown the wind, the argument goes, the party now reaps the whirlwind.
Others, however, say that Trumpism, no matter how much it threatens the existence of the modern-day Republican Party, is a broader manifestation of the uneven impact of globalization on a significant segment of the population, a rejection by these voters of institutions and elites in both parties, whom they see as having failed to listen to or respond to their plight.
In reality, it is both, a problem that has had implications for both major parties over a period of years but that has become particularly acute for the Republicans at this moment because the party so badly needs those voters to win in November.
One can scroll back over a half-century to find reasons or explanations for the rise of Trump. The Republican Party has long been engaged in recurring struggles between its long-dominant establishment wing and various embodiments of an anti-establishment, conservative insurgency seeking to upend the status quo.
Goldwater won that battle for the nomination in 1964 over the eastern elites, led by then-Gov. Nelson Rockefeller of New York, but split the party and went down to a crushing defeat in the general election. Out of the ashes came Ronald Reagan, who though twice elected governor of California, nonetheless was long viewed with disdain by the party's eastern elites.
Reagan's challenge to then-President Gerald Ford in the 1976 primaries represented the next great anti-establishment challenge by the party's conservative wing. That battle went all the way to the national convention in Kansas City, where Ford prevailed. When Ford lost the election to Jimmy Carter, Reagan and his anti-establishment conservatism laid claim to leadership of the party and eventually to the presidency.
In the eyes of anti-establishment Republicans, the election of George H.W. Bush in 1988 restored the establishment wing to power. Within two years there was another revolt, this one led by then-representative Newt Gingrich when Bush abandoned his pledge not to raise taxes as part of a controversial budget deal with the Democrats.
The 1990 rebellion contributed to Bush's defeat to Bill Clinton in 1992. Two years after that, the Gingrich-led forces swept to power in the House in 1994, Gingrich became Speaker, and the balance again swung away from the establishment.
Six years later, after Gingrich and his revolution faltered and he had left the speakership, the establishment reasserted itself when Texas's then-Gov. George W. Bush was elected president. "We reverted to the norm and the old order came back again," Gingrich said.
Many of these battles pitted familiar wings against one another: moderates vs. conservatives; the business wing vs. the evangelical wing; the mainstream wing vs. the populist wing. Many assumed that this year's Republican contest would be a rerun of contests between the establishment and a conservative insurgency. Trump's candidacy has scrambled those assumptions and for what are now obvious reasons - the changing composition of the Republican coalition.
In the 1970s and 1980s, white working-class voters in the north fled a Democratic Party they saw as too liberal on cultural and racial issues and migrated to the Republicans. Once a linchpin of the Democratic coalition, they later were dubbed Reagan Democrats, but the migration began long before his presidency. They joined white Southern conservatives who had earlier defected from the Southern Democratic Party to become Republicans.
Many of these white, working-class voters coexisted uneasily with the party establishment and at times with the purer strains of conservatism. "The white working class left the Democratic Party because it concluded that the party was committed to groups and objectives that were inimical to their economic interests," said William Galston, a Brookings Institution scholar and former White House domestic policy adviser in the Clinton administration. "The Republican Party promised to do better, but it never delivered."
Trump wasn't the first to tap into this anger. Galston points to the 1992 and 1996 presidential candidacies of Patrick J. Buchanan. The conservative commentator challenged Bush in the 1992 primaries and in 1996 led a populist revolt he described as "peasants with pitchfork." He ran on a platform similar to that of Trump today: anti-free trade, tough on immigration and focused on the plight of the white working-class ethnics.
"Trump stood up and said in effect [to the white working class], these Republican elites, they haven't done squat for you," Galston said." If you want someone who will stand up and defend your values and interests, here I am."
Henry Olsen, a fellow at the Ethics and Public Policy Center, sees the Republican Party leadership's failure to address the conditions of the white working class as the central reason for Trump's rise.
"They've been ignoring the economic pressures that have been placed on the native born, low-skilled person for the last 15-20 years," Olsen said. He added, "Trump walks into this and says, 'I'm an American first, I'm a conservative second. . . . We need to give the people who have been shafted for the last 15 years a leg up."
The Republican Party was absorbing these voters as part of its coalition at a time when economic conditions pressed harder and harder on their financial well-being, and the gap between the wealthiest and the rest of society widened, resulting in a hollowing out of the middle and working classes.
A Pew Research Center study late last year found that the share of income accounted for by the middle class has plummeted since the 1970s. The past 15 years have been especially harmful to the economic standing of these Americans, thanks to the recession of 2001 and the financial collapse of 2008.
Meanwhile, Republican economic orthodoxy continued to prescribe tax cuts that gave the biggest benefits to the wealthiest, arguing that this was the most effective way to stimulate economic growth, which in turn would help all segments of the population.
Katherine Cramer, a political science professor at the University of Wisconsin in Madison, has been conducting extended conversations with people in her state since 2007 as part of her studies of public attitudes and opinions. The expressions of anger and frustration heard at Trump rallies represent the kind of resentment she has heard for years.
"I think of them as folks in the middle, not devout Republicans, but people who are feeling economically pretty stressed," Cramer said. "I see the Trump phenomenon coming out of rising income inequality and the leftovers of the Great Recession. They are feeling unheard and kind of disrespected by typical powers that be."
Trump's constituency is not limited to the white working class, but it is within that group that he draws his strongest support, according to exit polls from the primaries this year.
"It's a sector of the electorate that is not particularly guided by the strong ideological positions as you find in Washington think tanks or as the intellectual elite like to think of the electorate," Ceaser said. "The ideology is malleable. If you go for ideology, you're going to miss the phenomenon."
Trump isn't the first Republican candidate with a populist economic message. Former Arkansas governor Mike Huckabee and former Pennsylvania senator Rick Santorum touched on similar themes in their campaigns. Neither, however, had Trump's style or bluntness.
Beyond economic issues, Trump has tapped fears about a changing America, a country that is increasingly diverse and culturally tolerant. These voters see the deterioration of values they believe are essential to the character of the country.
Trump also has spoken to continued worries about terrorist threats. It was perhaps no accident that, after the attacks in Paris and San Bernardino, Calif., Democratic strategists saw in their focus groups a spike in support for Trump.
"It is a group of people who are uncomfortable with changes in the social order or threats from the outside world," said Marc Hetherington, a political science professor at Vanderbilt University. "They favor strong leaders and aggressive tactics to snuff these threats out."
The Republican establishment has reacted with alarm at the possibility of Trump winning the nomination, with no better example than the unprecedented attack by the party's 2012 nominee, former Massachusetts governor Mitt Romney, on the man who could lead the party into the fall campaign.
But should Republican leaders be so surprised? Or were they agents in the rise of Trump? Robert Kagan of the Brookings Institution wrote an opinion article in The Washington Post in which he described Trump as a "Frankenstein monster" created by the GOP who now threatens to take over and possibly take down the party.
"What did Trump do but pick up where they left off, tapping the well-primed gusher of popular anger, xenophobia and, yes, bigotry that the party had already unleashed?" he wrote.
Democrats in particular see a racial component in Trump's appeal, which they say has intensified since Obama was first elected. When he toyed with running in the last election, Trump played to those attitudes by questioning whether Obama was American born and a Muslim, not a Christian.
Hetherington said racial attitudes have become more polarized during Obama's presidency. "There's a distinction to be made between racism and racial resentment," he said. "You can be resentful without being a racist." But he added there has been an "incredibly stark movement among Republicans" in the direction of greater racial resentment.
"Sentiments like this are partly there, partly not there," Ceaser added. "The tragedy of Trump is he's taken all these things that are embryonic and brought them to the forefront."
Vin Weber, a former House member from Minnesota and a longtime GOP strategist, challenged that assertion. "The notion that what's going on is somehow the result of misguided or nefarious leadership of the top is just a misunderstanding of what's going on," he said, noting that the GOP establishment fought the tea party in a series of Senate primaries in 2014.
Weber did find fault with his party, saying it has tried to destroy the concept of legitimate government. "We should be the party of limited government," he said. "But we've gone beyond that and allowed much of our rhetoric to say that government is just evil and the people working in it are evil."
Gingrich, who felt the sting of the Republican establishment in 2012 when he was challenging Romney in the GOP primaries, sees those condemning Trump and his followers as blind to the genuine anger around the country - at Obama in particular and also at the Republican leadership.
"You have a party that mishandled the economic collapse, an elite leadership that failed to reform things, an attitude of arrogance to the very tea party people who wanted to change things," he said. "The people in the imperial capital cannot understand why everyone in the rest of the country is offended by sending money to the imperial capital."
Gingrich said he believes Trump could become an effective ally of conservatism as a reform president. But to many in the party, Trump and his following represents what Galston called "the first popularly based challenge within the Republican Party to basic conservative premises since Reagan.
That's why so many in the party are so worried at this moment. "There is nothing trivial about this, and it may result be a real rending of the party," Weber said. "But I don't think it means the death of the party. If worst comes to worst, which I hope doesn't happen, I think the recovery will be faster than people think."