It was a big moment for then-Rep. Chris Collins when he took the national stage to officially nominate Donald Trump as the Republican presidential candidate at the party’s 2016 convention.

Three years later, Collins pleaded guilty to insider trading and lying to the FBI, and he now faces a 26-month prison sentence.

It was a long fall for one of Mr. Trump’s prominent backers, but he was hardly the only one from that Cleveland stage who has faced hard times since the convention.

Jeff Sessions, Joe Arpaio, Ryan K. Zinke and David Clarke have all had their political careers derailed. Actor Antonio Sabato Jr. says he can no longer get work in Hollywood.

Jerry Falwell Jr. was recently pushed into taking leave as president of Liberty University, the school founded by his father.

Michael Flynn made it just three weeks as White House national security adviser before he was fired for lying to Vice President Mike Pence, and is now battling to recant his plea of guilty of lying to the FBI.

Chris W. Cox was ousted as the No. 2 staffer at the National Rifle Association, emblematic of the chaos that has ripped through the gun rights group since the 2016 election.

“What revolutions all have in common is revolutionaries end up getting squashed,” said Michael McKenna, a former top legislative aide in the Trump White House who now writes a column for The Washington Times.

Call it the Trump convention curse.

A striking number of those who were willing to stand up and be counted backing Mr. Trump in 2016, when many Republicans and others shunned him, have struggled in the ongoing insurgency.

“Trump always, because he himself was such an unusual outside-the-box candidate, attracted a lot of people who were just different, unconventional,” said David Greenberg, a political historian at Rutgers University. “It doesn’t surprise me that when you get into more eccentric types, you’re going to get people who engage in more eccentric or perhaps illegal behavior.”

The 2020 Republican National Convention kicks off Monday and, like the 2016 edition, will feature an eclectic mix of personalities.

Sprinkled among Trump family members and Republican politicians are people who became social media sensations by clashing with cancel culture.

That includes Nick Sandmann, the student from Covington Catholic High School in Kentucky whose encounter with an American Indian man at the Lincoln Memorial became a Rorschach test for the Trump era, and the McCloskeys, a St. Louis couple who held off Black Lives Matter protesters with guns.

The Trump team said they hope to provide a contrast with the gripefest they said Democrats delivered at their convention last week.

“There’s going to be some breakout stars, some people that you would not expect to be supporters of the president, and it’s going to tell a very beautiful story,” Trump campaign adviser Jason Miller told NBC’s “Meet the Press” on Sunday.

Four years ago, Mr. Trump did try to field a star-powered lineup with a special emphasis on National Football League stars.

But attempts to get quarterback Tom Brady fell flat; former football coach Mike Ditka, though a Trump supporter, declined to speak; and Tim Tebow was on an early leaked list of names but quickly made clear he wasn’t going to be speaking.

One Orthodox Jewish rabbi slated to give an invocation also withdrew, saying his appearance had become a political dividing line.

Mr. Trump ended up with an eclectic set of speakers, including figures from the 2012 terrorist attack in Benghazi, Libya; pro golfers; and actors Scott Baio and Mr. Sabato.

Mr. Sabato told Variety this year that he can no longer get a job, despite three decades of acting experience, because of his speaking role.

“It’s been terrible. It’s mind-blowing. It’s a disgrace. It’s tough, because if you’re in that environment in Hollywood and you have something to say that they don’t like, they’re going to let you know,” he told the publication.

He made an ill-fated run for Congress in 2018 and has since had to work construction jobs to pay the bills.

Mr. Zinke, at the time a congressman from Montana and a former Navy SEAL, was a key voice for Mr. Trump at the 2016 convention and was rewarded by being tapped for the post of interior secretary.

Two years later, and facing a series of investigations, he called it quits.

“I’m trying to put D.C. behind me. I don’t miss it,” he told a Montana gathering this year.

Mr. McKenna said it has always been a struggle for this president to find people skilled enough to serve in government while disaffected enough to want to shake it up the way Mr. Trump does.

“What you wind up getting is people [whose] ability to maneuver around the institution is not as good as it should be, or people whose commitment to the disruption is not sufficiently firm,” Mr. McKenna said.

Mr. Sessions seemed like a man who could straddle that line, with decades of Washington experience and with a commitment to the same policies Mr. Trump championed on immigration. He was so taken with Mr. Trump that he was the first senator to endorse the maverick businessman candidate in 2016.

Mr. Trump rewarded him by nominating him to be attorney general. But after two years of public sniping, Mr. Sessions was out. Mr. Sessions tried to regain his Senate seat from Alabama this year, but the president helped sink his candidacy by opposing him in the Republican primary.

Historians struggled to draw parallels to the Trump convention curse.

“I can’t think of another situation remotely like this in history,” Mr. Greenberg said. “What seems different about Trump in 2016 was how many mainstream Republicans either refused to endorse him or, at a minimum, avoided the convention. So to fill the time, they had people like Clarke and Scott Baio. The eccentric roster probably had something to do with their later sordid fates.”

Stan Haynes, who has written two books on political conventions, said there have been scandal-plagued administrations, but in the 19th century — which is the focus of his books — conventions were chiefly internal party affairs.

Prospective nominees didn’t even attend — “it was viewed as unseemly,” Mr. Haynes said — and speakers were senators, congressmen and governors.

Trump backers bristle at the sense that his list of advisers is more tainted than those of other presidents.

In particular, they point to Flynn, a former Army lieutenant general who clashed with President Obama and joined the Trump campaign.

Mr. Trump made him national security adviser, but Flynn lasted just three weeks at the White House after admitting he lied to the vice president about having spoken with the Russian ambassador during the presidential transition.

Special counsel Robert Mueller later brought charges accusing Flynn of also lying to the FBI. Flynn pleaded guilty but has tried to withdraw the plea, and the Justice Department has sought to drop the case. A federal judge is battling to block those moves.

Trump backers say Flynn was railroaded and call it an example of the unfair treatment this president’s team has faced.

To be sure, most of the 2016 speakers remain enthusiastic backers of Mr. Trump. Mr. Sabato peppers his social media with praise for the president.

Even Mr. Sessions, whose political comeback was torpedoed by Mr. Trump, said on the campaign trail that he still backed the president.

Perhaps the most curious speaker in 2016 was Andy Wist, owner of Standard Waterproofing, a business he said he started at 21 in his mother’s backyard in Brooklyn, and grew to 140 employees.

He said Mr. Trump would build the kind of economy where companies like his could flourish.

Mr. Wist didn’t respond to an inquiry last week from The Times.

But the convention curse doesn’t appear to have struck him. A couple of months after his speech, Mr. Wist added a news agency photo of himself speaking at the convention, in blue suit and red power tie, to his company website. It has remained there since.