The volatility of the Republican presidential race threatens to undermine the ­party’s July convention, putting potential donors on edge, raising security concerns and prompting some GOP politicians, including those in competitive reelection battles, to skip the Cleveland gathering altogether.

A bungled and possibly contested convention could have lasting repercussions not only for the eventual nominee but also for the Republican brand. Party leaders fear that a week of contentious floor fights, inflammatory rhetoric and potentially violent protests could project a negative image to voters nationwide.

Compounding the challenges facing organizers are the expectations of Donald Trump, who asserted in an interview that he should have at least partial control over programming, stagecraft and other issues by virtue of his front-runner status — even if he does not have the delegates to secure the nomination beforehand.

Trump blasted the GOP’s last convention, in Tampa four years ago, as “the single most boring convention I’ve ever seen.” The billionaire real estate mogul and reality-television star said it was imperative that this year’s gathering have a “showbiz” quality — and he cast doubt on the ability of the Republican National Committee, which oversees the convention, to deliver.

“It’s very important to put some showbiz into a convention, otherwise people are going to fall asleep,” Trump said in a 45-minute interview here last week in his Trump Tower office. “We don’t have the people who know how to put showbiz into a convention.”

Trump’s comments capped a week of feuding between him and the RNC as Sen. Ted Cruz of Texas has outmaneuvered him in the chase for the convention delegates who will decide the nomination. Trump argued that the delegate selection process is “rigged,” prompting the RNC to proclaim in a memo that the rules of the system have been clear.

Trump left open the possibility that he would seek to install his own allies at the RNC should he accrue the 1,237 delegates required to win the nomination by the time primary voting ends in June. Asked in the interview whether he would retain RNC Chairman Reince Priebus in that scenario, Trump replied: “I don’t know. I haven’t made the determination.”

Despite the sparring, some party leaders say they remain optimistic about putting on a successful convention. RNC members will meet this week in Florida and receive updates on planning for Cleveland and political preparations for the general election.

“There’s an unprecedented level of excitement for our party and our convention,” said the RNC’s chief strategist, Sean Spicer. “We’re going to have an opportunity to present our candidates and our principles not just to the country but to the world, and I think it’s going to propel us to victory in November.”

Rep. Steve King (R-Iowa), national co-chairman of the Cruz campaign, said that “the [convention] program should be set by a balanced mix of the candidates and the RNC. . . . You don’t want to give Trump a blank check to run the convention.”

And John Weaver, chief strategist for the Kasich campaign, dismissed the idea that Trump should decide programming, absent a majority of the delegates. “He’s not the nominee. He can decide anything if he gets to 1,237. But until then, no,” Weaver said. “If you’re ahead in the third quarter of the Super Bowl, you don’t get to decide who gets the Lombardi Trophy.”

If the convention is contested, one factor up in the air is the vice presidential nomination: Would Trump, Cruz and Ohio Gov. John Kasich each arrive in Cleveland with declared running mates? Trump said he plans to name one only if he has the nomination secured; otherwise, he will wait to do so at the convention.

Trump supporters argue with protestors outside the David L. Lawrence Convention Center in Pittsburgh, where Republican presidential candidate Donald Trump had just spoke on Wednesday, April 13. (Jabin Botsford/The Washington Post)

Meanwhile, the city of Cleveland is preparing for potential violence. Over the past two months, the city has sought bids for 2,000 sets of riot gear, knee and shin guards, breastplates, and other protective items for its police officers, as well as flexible handcuffs, collapsible batons and miles of interlocking steel barriers.

The equipment will be funded through a $50 million federal security grant.

“We are excited and we anticipate a great convention,” said Daniel Williams, a spokesman for Cleveland Mayor Frank G. Jackson (D).

Trump has privately voiced displeasure with the RNC’s selection of Cleveland, a heavily Democratic city, believing it could become a destination for people looking to disrupt the Republican celebration, according to people who have spoken to him.

In the interview, Trump said with irritation, “It should be a monumentally magnificent convention and it should be brilliantly staged, but they’re spending $50 million on security.”

Philadelphia, where the Democrats will hold their convention later in July, also is getting a $50 million federal grant to cover security costs.

Last month, Trump predicted that if he arrived in Cleveland roughly 100 delegates shy of the threshold and is not easily handed the nomination, “I think you’d have riots.” He has since backed off talk of violence, but Roger Stone, a Trump confidant who is not part of the campaign, is organizing supporters as a force of intimidation.

In recent interviews, Stone has previewed a “day of rage” and threatened to publicly disclose the hotel room numbers of delegates who work against Trump. Stone’s activist group, Stop the Steal, has a website advertising “four days of non-violent demonstrations, protests and lobbying delegates face to face. . . . We must own the streets. In numbers there is strength.”

Priebus said Sunday on CBS’s “Face the Nation” that Stone’s talk was not “appropriate,” adding: “I’m committed to making sure that the delegates have a great week — that they have a fun week, but a constructive week and a safe week.”

This summer's political conventions could get heated – but it certainly wouldn't be the first time. (Peter Stevenson/The Washington Post)

The potential for chaos in Cleveland — not to mention associating their brands with Trump’s divisive candidacy — has led some corporations and other donors to reevaluate whether to sponsor the convention.

“If you’re not a longtime participant, this is not the year you’re going to start,” said Stewart Verdery, a GOP lobbyist. “Others are scaling back or waiting longer. If you add it all up, there’s a drop-off in commitment. But it’s not some drastic kind of thing.”

The RNC says it had raised nearly $12 million for its convention committee through March 31.

Unlike in past years, the conventions will not receive public funding, putting the pressure on the parties to come up with all the cash to finance the events. In 2012, each convention committee spent about $18 million.

Separately, the Cleveland 2016 Host Committee is charged with raising tens of millions of dollars to produce the festivities around the convention. Typically, those costs are covered by big corporations, along with some individuals who ante up large sums.

“The Cleveland 2016 Host Committee has raised $56 million of our $64 million goal — which is more money raised than any other convention in history,” spokeswoman Emily Lauer said. “Our fundraising is on track, and we continue to make forward progress. We’re confident that we’ll raise the $64 million needed to host a successful convention in Cleveland this summer.”

For corporate lobbyist donors, one perk of the convention is to mingle with elected officials. But the list of Republicans planning to skip the convention is growing.

Former Florida governor Jeb Bush, a Trump critic and unsuccessful 2016 presidential candidate, will not go. Sen. Mark Kirk, facing a tough reelection challenge in blue-state Illinois, announced last week that he would not attend, while Sens. Kelly Ayotte (N.H.) and Richard Burr (N.C.) told CNN that they probably will stay away.

Sen. Rob Portman (Ohio), also on the ballot this November, will attend the convention in his home state but will mostly be involved in outside “volunteer appreciation events” for his Senate campaign and a local Habitat for Humanity project, campaign manager Corry Bliss said.

Mitt Romney, the 2012 presidential nominee, who gave a blistering anti-Trump speech last month, did not respond through a spokeswoman when asked whether he would be in Cleveland.

Trump said in the interview, “I think it’s very hard for Bush and some of these guys to come back and feel well toward Trump, feel good about Trump. Romney, the same thing. I don’t know if I can bring Romney back.”

Later, however, Trump said he would have no qualms about Romney being in Cleveland: “I don’t care. He can be there if he wants.”

Trump charged that Romney made a serious error in 2012 by refusing to air a convention video Trump had filmed in which he sat across his office desk from a Barack Obama look-alike.

“I said, ‘Barack Hussein Obama, you’re fired!’ ” Trump recalled of the video, which was a nod to his NBC show, “The Apprentice.” “But [Romney’s campaign] never played it. They thought it was too controversial. Stupid people. The cinematographer said it was one of the best things he ever did.”

In Cleveland, Trump envisions this kind of “showbiz” element. He wants to give business leaders and other non-politicians speaking slots. Asked, for instance, if he would support giving the keynote slot to Gov. Nikki Haley of South Carolina — one of the party’s shining stars and a Trump critic — Trump said, “Nikki Haley would not be my first choice.”

At this stage in recent past cycles, a presumptive nominee had emerged and begun orchestrating the convention around his campaign. But if the Cleveland gathering is contested, it will fall to Priebus and his RNC lieutenants to put on the show.

Former RNC chairman Michael Steele said party officials are likely to devise broadly acceptable themes, such as “rebuilding America” and “unity,” that could be customized at the last minute to suit the eventual nominee.

“The next few months will be about coming up with things that won’t offend Trump, Cruz or Kasich,” Steele said. But, he added, “If Trump is sitting 60 to 80 delegates away from the nomination, he may be the only one in position to make significant demands — and they may have to give him the deference he wants.”

Costa reported from Washington. Matea Gold, Tom Hamburger and Catherine Ho in Washington contributed to this report.