Work continues on the main stage for the Republican National Convention, Tuesday, June 28, 2016, in Cleveland. (Mark Gillispie/AP)

Dozens of well-known Republicans aren’t showing up. There’s no word yet on who will speak. A growing number of corporate sponsors are taking a pass. Groups of white supremacists and other agitators are on the way, while the official protest routes are frantically being redrawn after being thrown out in court. And then there’s the fight to dethrone the big star.

With less than three weeks to go, Donald Trump’s Republican National Convention in Cleveland is poised to be the most chaotic GOP gathering of the modern era.

The candidate, his family and close supporters are expected to play starring roles. So will most top congressional leaders. But many Republicans who want to distance themselves from Trump’s incendiary rhetoric are refusing to attend. Past corporate sponsors such as Ford, General Electric and JPMorgan Chase have declined to participate.

The four-day meeting kicks off July 18 at a downtown multipurpose arena with Trump scheduled to formally accept the nomination July 21. Convention organizers denied reports that several sports figures would be speaking at the convention. There’s still no official word on who will speak or entertain the delegates. Lee Greenwood, who has performed his hit “God Bless the USA” at several GOP conventions, declined through a spokeswoman to say whether he’s attending. Singer Ted Nugent, a Trump fan, is skipping the convention despite numerous invitations to appear “due to our intensive concert touring schedule,” a spokeswoman said.

Trump has called for a glitzier affair, telling The Washington Post in April that “it’s very important to put some showbiz into a convention, otherwise people are going to fall asleep. We don’t have the people who know how to put showbiz into a convention.”

With the GOP convention less than two months away, Cleveland scrambles to prepare for the arrival of tens of thousands of delegates, demonstrators and journalists. The Washington Post's Ed O'Keefe walks us through the latest developments to see where the city stands ahead of the convention. (Dalton Bennett/The Washington Post)

Shortly after Trump clinched the nomination in early May, convention organizers began sharing plans for the event’s staging and schedule with his aides, who traveled to Cleveland to inspect the facilities and begin reviewing details. A stage design unveiled on Tuesday — a black circular platform surrounded by white stairs with a large video backdrop — was reviewed personally by Trump who requested a few changes, according to convention officials.

One convention spokeswoman said it was “very premature” to conclude that the event might be troubled, adding that programming details have yet to be announced.

But Republican delegates concede that this year’s convention will be different.

“This is a volatile year, and if we have learned anything so far, it is that the customary rules and methods of winning elections probably don’t apply,” said Steve Duprey, a delegate from New Hampshire.

Before completely focusing on presumptive Democratic nominee Hillary Clinton, Trump first must quell a potential insurrection at his own convention. He’s preparing a team of 150 staffers and volunteers designed to corral votes, push potential changes to the party’s platform and, most important, block any attempt to unseat him.

A plan to allow convention delegates to vote however they want, rather than follow the results of their state’s primary, has earned the support of hundreds of delegates upset by Trump’s impending nomination, according to Free the Delegates, the group pushing for the change.

But a five-member Trump “study committee” is focused on quashing any effort to unbind delegates. According to people familiar with the plans, the quintet includes Trump campaign attorney William McGinley and members of the convention rules committee: Bill Palatucci, a Republican National Committee representative from New Jersey; Alex Willette, an RNC committeeman from Maine; Demetra DeMonte, an RNC committeewoman from Illinois; and Vincent DeVito, a Trump supporter from Massachusetts.

Separately, delegates in Colorado, Louisiana, Ohio and elsewhere report being contacted by an unknown entity — some suspect the Trump campaign or the Republican National Committee — conducting a telephone “push poll” with questions about their support for Trump, a possible convention rules change and other related issues. Representatives of the Trump campaign and the RNC did not return requests for comment.

The pro-Trump pushback appears to be working. Free the Delegates is struggling to win votes on the 112-member rules committee, which is scheduled to consider the group’s proposal in the week before the convention.

“The majority has spoken in favor of Mr. Trump,” said Rosie Tripp, a rules committee member from New Mexico. “Therefore, my vote ‘must’ represent them and not me or my opinion.”

Diana Orrock, a rules committee member from Nevada, faulted some delegates for trying “to change the rules of the game in the middle of the process.” She dismissed Free the Delegates as “representing themselves to be far more influential and a much bigger movement than they are.”

But Kendal Unruh, the leader of the group, said it is clearly succeeding if Trump feels compelled to divert resources and focus on defeating the effort. She said an updated tally of the group’s support would not be available until Thursday.

“We’ve got a candidate who lied to everyone and said he’s going to self-fund,” Unruh said, referring to Trump’s paltry campaign coffers. “I’m sure he has more staff at the convention than he does on his national campaign.”

Unruh and her team are raising money through a separate PAC that is planning to hire staff and rent office space in Cleveland to serve as a command center.

Steve Lonegan, a New Jersey-based GOP consultant advising the group, cited several recent polls that put Trump behind Clinton, in some cases by double digits.

“If Trump reverses that trend in the next two weeks and galvanizes the party, then our effort will diminish, we realize that,” Lonegan said. “But as of this week, Donald Trump appears to be poised to take the Republican Party down to a disastrous defeat with major down-ballot consequences.”

Democrats, meanwhile, are scheduled to meet July 25-28 in Philadelphia, a city that has hosted several political conventions. While protesters opposed to Clinton and supporting Sen. Bernie Sanders (I-Vt.) vow to show up, delegates inside the convention hall are expected to be much more united behind their nominee.

The anticipated chaos in Cleveland has dissuaded some vulnerable Republicans and once-generous corporate sponsors from showing up.

Members of Bush family, including the former presidents, are planning to skip the convention even though they essentially helped build the modern GOP. Mitt Romney, the party’s 2012 nominee, also won’t be there.

Many current GOP leaders remain committed to going. House Speaker Paul D. Ryan (Wis.) will attend as the convention’s official chairman, and all of his lieutenants are scheduled to be there, including House Majority Leader Kevin McCarthy (R-Calif.), who is a Trump delegate. Senate Majority Leader Mitch McConnell (R-Ky.) will be at the convention with most of his leadership team in tow.

But Sen. Roy Blunt, the fourth-ranking Senate Republican, will be in Missouri tending to a closer-than-expected reelection campaign. Several other incumbents up for reelection are also steering clear, including Sens. Kelly Ayotte (N.H.), John McCain (Ariz.), Patrick J. Toomey (Pa.) and Ron Johnson (Wis.). Two who will be there are home-state Sen. Rob Portman (Ohio) and Sen. Marco Rubio (Fla.), who said he is willing to speak at the convention.

Some vulnerable GOP House incumbents are also skipping, including Reps. Mia Love (Utah), Elise Stefanik (N.Y.) and Carlos Curbelo (Fla.).

In 2012, companies including Amgen, General Electric and Ford ponied up tens of thousands of dollars to officially sponsor the Republican convention in Tampa. This year, they cite varying reasons for withholding support; none mention Trump.

Several traditional sponsors are still involved, including Google, which is serving as the convention’s “official live stream provider.” Coca-Cola donated $75,000 each to the Republican and Democratic conventions, a smaller sum than previous years. The American Petroleum Institute is also still on board: “Because our candidate is energy, we are supporting both conventions,” a spokesman said.

David Gilbert, president and chief executive of Cleveland’s 2016 convention host committee, said he is about $6.5 million short of his group’s $64 million goal.

“The tenor of this political cycle has caused some effect, but I would consider it a relatively minimal effect and should not stop us from meeting every financial obligation that we’ve got,” he said in an interview.

Groups planning demonstrations for and against Trump are still sorting out where exactly they can march. A federal judge ruled that protest routes planned by Cleveland officials were unconstitutional and ordered up a new plan. Talks are underway among city leaders, federal security officials and protest organizers, but there’s no way to ensure that pro- and anti-Trump factions won’t come face to face in the streets of Cleveland, potentially causing mayhem.

Officials are particularly concerned about extremists on either side. A group of white nationalists who held a rally in California last weekend where five people were stabbed has said it plans to show up in Cleveland “to make sure that Donald Trump supporters are defended,” according to a McClatchy report.

“I think there may be chaotic parts, and I think there could be instances of violence. But I think most of the time things will run smoothly,” said Christine Link, executive director of the ACLU of Ohio, which sued the city on behalf of protest groups.

Some delegates say they’ve discussed potential security risks, but received assurances that there will be enough security to protect them. Those who dislike Trump still plan to show up to deal with other party business.

“This isn’t going to be one of those things where we back 100 percent of what our candidate says,” said Craig Dunn, a delegate from Indiana. “The only one real consistent thing is that what unites us is that we believe Hillary Clinton is a thoroughly corrupt person.”

Robert Costa contributed to this report.