The Quicken Loans Arena is seen as setup continues in Cleveland. (Aaron P. Bernstein/Reuters)

Thanks to Donald Trump, the Republican National Convention that opens here Monday will be like no other in the modern era — a gathering of a divided and nervous political party preparing to nominate a candidate who stormed through the nominating process after turning his back on a generation or more of conservative orthodoxy.

In many ways, what transpires in Cleveland will seem familiar. There will be the customary symbols of political conventions — a series of speeches, goofy hats and pins, balloon drops, and relentless attacks on the opposition party and its nominee. GOP leaders will attempt to project at least a patina of unity to the worldwide audience that will be tuning in.

Yet there will be no hiding the obvious — that an alternate reality forms the true backdrop for this convention, with many Republican leaders worried about what Trump’s candidacy has done to break apart their coalition and what he might do to their overall fortunes in November.

The modern Republican Party has been shaped by many forces, the most important being the presidency and conservative philosophy of Ronald Reagan. It was Reagan who cast modern conservatism in a positive and optimistic light and who moved the party sharply to the right after the debacle of Barry Goldwater’s defeat and the presidencies of two later Republicans, Richard Nixon and Gerald Ford.

Other politicians, movements and events have shaped it since, to the point that Reagan might not recognize — or be welcome in — the party of today. After Reagan came the presidency of George H.W. Bush, which saw an end to the Cold War but foundered domestically. A backbench rebellion in the House, led by Newt Gingrich, helped shorten that Bush presidency but brought the GOP to power in Congress. After Gingrich came a second Bush presidency, that of George W. Bush, who after the attacks of Sept. 11, 2001, launched a disastrous war in Iraq that divided the country.

Here are some of the people who are speaking at the 2016 Republican National Convention in Cleveland and some who've opted to skip the event. (Sarah Parnass,Danielle Kunitz/The Washington Post)

During President Obama’s tenure in office, Republicans have experienced a grass-roots tea party revolt, grand successes in midterm elections that brought the party to a high point of power in the states and consecutive failures in the past two presidential elections that underscored long-term vulnerabilities of a predominantly white party in an increasingly diverse country.

Throughout this period of change, through victories and defeats, the Republican Party and its followers adhered to a set of small-government, pro-defense, socially conservative principles that were consistent and as coherent as any political coalition can muster. Now, in the course of one tumultuous year, Trump has shattered that consensus, exposing divisions that were either overlooked or ignored by the party establishment. As Ohio Gov. John Kasich put it in a recent interview, “I think the party right now is trying to figure out what it is for.”

Trump has changed, at least for now, much of what everyone believed it meant to be a leader of the Republican Party. He is anti-trade in a party of free traders. He wants to keep Social Security and Medicare mostly as they are, while others in the party are committed to entitlement reform. He has questioned America’s role in the world in a party long dominated by internationalists and, more recently, interventionists. He has spoken about the LGBT community in ways many Republicans do not. He has put the establishment on notice and ignored its advice when it suits him — which is most of the time.

The very fact Trump holds those views rankles many in the party. What adds to their discomfort is that — while espousing those views — he managed to win more states, more votes and more delegates than any of the other 16 Republicans running for president.

The ruptures caused by Trump’s candidacy will be felt in Cleveland, if not always seen. What is left to be answered is whether Trump’s impact is lasting. Or will his candidacy prove to be a brief, if unnerving, episode that fades quickly if he loses in November, allowing the party to return to some semblance of normalcy?

Those alarmed at the prospect of Trump at the top of the ticket in November despair at the state of the party. “I think it’s incredibly divided,” said Katie Packer, a GOP strategist who led a super PAC that tried to deny Trump the nomination. “You have Republican-on-Republican aggression because people are arguing over politics versus principle. Do we stand by the party that we’ve all been loyal to, no matter what, or do we stand up and say, ‘No, this behavior is unacceptable, and I want no part of it under any banner’? It’s put people that they’re used to being in the bunker with against one another.”

But defenders of the presumptive nominee have another view of his impact on the party. Pollster Kellyanne Conway, who is now part of the Trump campaign, said the primaries highlighted fissures within the Republican coalition between what she called the political and voting classes and changed the balance of power between them. “This is the year the voters took the party back,” she said.

Conway, who worked during the primaries for a super PAC supporting Sen. Ted Cruz of Texas, said it was telling that Trump and Cruz finished as the top two candidates in a large field of experienced insiders. “The Republican Party was veering dangerously close to cementing itself as the party of elites,” she said. “I would look at it as the haughty versus the rowdy in our party. . . . I’m not assigning any negative attributes to that. They’re [the Trump supporters] frustrated. They’re fed up and they feel betrayed.”

How splintered is the party? Over the past few years, a number of authors have published books examining the state of the Republican Party. Although they have different perspectives and range across the ideological spectrum, three of these writers, in recent interviews, came to similar conclusions: The party circa summer 2016 is in a predicament, partly of Trump’s doing and partly the result of other forces, with no clear or simple way out.

“I think it’s in bad shape. I think it was in bad shape before,” said Matt Lewis, a conservative who writes for the Daily Caller and who wrote the book “Too Dumb to Fail.” “Trump in some ways has just exploited the lack of coherence, of competence within the party. I think he’s created chaos, but he’s also exploited preexisting conditions.”

In Lewis’s analysis, the party has been unraveling in one form or another since the end of the Cold War. Anticommunism was the glue that bound together a coalition of conservative, moderate and liberal Republicans who often saw the rest of the world differently. “I think now you’ve gotten to the point where there is a huge divide between the base of the party, which I think identifies with Donald Trump — it’s nativist and populist — and intellectual leaders like [House Speaker Paul] Ryan [Wis.], [Sen. Ben] Sasse [Neb.] and [Sen. Marco] Rubio [Fla.], who are more optimistic and forward-looking.”

Geoffrey Kabaservice wrote “Rule and Ruin,” an examination of the decline and fall of the GOP’s moderate wing in the period from the Eisenhower presidency to the early part of this century. In an interview, he noted that his admiration for the party’s now withered moderate faction doesn’t negate the reality that the period in which it was most dominant was when Republicans were doing terribly in election competitions generally.

“Since turning to conservatism, Republicans have seen all their dreams come true, at least in Congress and state legislatures and governorships,” he said. “So in that sense, the party is doing great.”

But as he was quick to add, that is only part of the story, the other half being the friction below the surface. “Trump . . . seems an expression of very negative things going on within the party that are now coming into fruition,” he said.

E.J. Dionne is a Washington Post columnist who critiqued the party from the perspective of a committed liberal. His book “Why the Right Went Wrong” examines the party’s evolution from Goldwater to the present day.

“One of the things Trump shows is that people vastly overread the tea party as some kind of pure libertarian anti-state movement and underestimated how many of its supporters were older, white Americans who were very angry about immigration,” he said. “Trump’s advantage was that he was outspoken about immigration, in an extreme and sometimes racist way, where many of his opponents felt constrained talking about it.”

He added that a key to understanding why the party has been so split by Trump’s candidacy is what conservative intellectuals have been noting all year. “The Republican Party relied for decades on white, working-class voters and delivered no material benefits to them,” Dionne said. “Oddly, a man who says he is a billionaire has become the avatar of a working-class rebellion inside the Republican Party.”

The gulf between the Trump party and the more traditional Republican Party can be seen by the list of Republicans who have withheld their support from the presumptive nominee. They include the three Bushes — the two former presidents and Trump rival Jeb Bush, as well as the party’s 2012 nominee, former Massachusetts governor Mitt Romney, who has led the anti-Trump forces since last spring. Many other prominent Republicans will be absent from Cleveland this week while some there will be less than enthusiastic about the presumptive nominee.

Many Republicans fear a possible debacle in November. Optimistic Republicans still see a path to victory for Trump, given the anger that animates parts of the electorate as well as long-standing hostility toward Hillary Clinton. GOP congressional leaders are prepared to do whatever they must do to preserve their majorities should Trump falter in the late stages of the campaign.

No matter the outcome, however, Trump’s candidacy has highlighted a party now debating what it actually believes. Vin Weber, a former House member from Minnesota and veteran strategist, said his concerns about the party go far beyond some of the offensive statements Trump has made.

“I’m as offended by that as anybody,” he said. “But, if this were just about the personality of Donald Trump, it would not cause me to say the party is in tough shape. . . . He is rejecting major, major policies that the Republican Party has stood for. . . . You have to ask that question: ‘What is it that we’ve misjudged about what the Republican Party actually believes?’ ”

Weber’s answer is that Republicans have let their hostility toward President Obama, rather than the principles of conservatism, shape what they think about issues. He worries that the anti-Obama sentiment that has bound the coalition together is morphing into anti-Clinton anger, and that raises the question of where the party actually stands on key issues.

Weber said a Trump victory in November could shatter the party. “I think it will cause a fracturing of the party more serious that we’ve seen up to now,” he said. “I can’t believe that the institutionalization of this phenomenon goes without severe consequences for the Republican Party.”

Dionne argued that Trump’s success should be a wake-up call to the Republican establishment that what the party long has preached has lost its resonance to many who have been voting Republican in recent years. “While Trump should have shown Republicans that the old small-government ideology does not speak to a lot of people who support the party, the congressional party . . . has not moved off those old positions.”

Conway said the success of the New York billionaire presents Republicans with a choice of what it will be in the future: “Cementing your status as a party of elites or following Donald Trump’s lead and become the party of the workers.”

Kasich doubts that the party will break apart but nonetheless sees Trump as emblematic of a broader — and to him worrisome — shift in attitudes here and elsewhere. “I am increasingly concerned about worldwide — not just in America but worldwide — growing nationalism, a movement towards anti-immigration, a movement towards anti-trade, a movement towards isolation. None of these things am I comfortable with for our country, not just my party, but my country.”

The Republican convention will hardly resolve the differences that Trump’s candidacy has revealed. They will continue to roil the party all the way through Election Day — and likely beyond.