Colin Dussault has sold about 2,000 of his "Greetings from Cleveland: It's Gonna Be a Riot" T-shirts ahead of the Republican national convention, which starts in the Ohio city on Monday.

The idea came to the 47-year-old musician, who is from the Cleveland suburb of Lakewood, when he awoke one Sunday in March after Donald Trump warned the Republican Party that there would be riots if they tried to stop him winning the party's presidential nomination.

"It is a tongue-in-cheek reaction to Donald Trump and the circus that is coming," he says. "It started out on Facebook with friends. I got a little bit of press, and since then I have been sending shirts off to West Virginia, Texas, Arkansas, all over the US."

In March Trump was well ahead in the Republican presidential primaries, but it was unclear whether he would secure the majority support he needed to be party nominee. The rise of the “Never Trump” movement and the growing opposition prompted him to make his prediction.

By early May Trump had sewn up the nomination with the backing of more than 14 million primary voters, but victory has come at a cost for the Grand Old Party. Inflaming the worst instincts of an electorate angry for change, the New York property developer’s incendiary remarks on race, religion and immigration have splintered the party, unveiled a grotesque side to national debate and tarnished a Republican brand that needed a radical makeover to appeal to voters whom it has alienated for years.

Trump needs almost five times as many voters as he won in the primaries to support him in November, and many Republicans regard this as at best an uphill task.

His appointment as the party’s standard bearer heading into the November presidential election may be little more than a formality in Cleveland, but that’s not to say there won’t be protests. “There will probably be some outside forces who want to come, and they will push their agenda. It’s possible that things could flare up. I am hoping it doesn’t,” Dussault says.

Cleveland should have been the city where the Republican Party laid out its future plans and a bold strategy to defeat Hillary Clinton and prevent a third successive Democratic term in the White House. Instead it has Trump and a fractured party.

The bright glow of victory radiating from LeBron James's Cleveland Cavaliers in the National Basketball Association championships last month – the Ohio city's first major sports title in 52 years – is unlikely to generate a warm, fuzzy feeling among thousands of Republicans who descend on the Quick Loans Arena next week.

For decades the quadrennial national conventions of the country’s political parties involved elders meeting behind closed doors in smoke-filled rooms to pick their standard bearer in that year’s presidential election. They have since become little more than coronations, made-for-TV pageants aimed at transforming party members into passionate campaigners for the prospective president and converting millions of television viewers into guaranteed voters.

While conventions are carefully scripted to maximise every televised second, violent protests, bizarre speeches, verbal gaffes and awkward photo ops can turn them into car-crash television. Trump's disdain for scripts and flair for the unpredictable mean that next week might throw up even more surprises. The billionaire was quick to deny on Twitter one report that he had invited the former heavyweight boxing champion – and convicted rapist – Mike Tyson to be a guest speaker.

Speakers lined up include Eileen Collins, the first woman to command a space-shuttle mission, and the Silicon Valley billionaire Peter Thiel, a founder of PayPal. Trump’s wife, Melania, and four children will have speaking slots, too, along with prominent party figures such as speaker of the US House of Representatives Paul Ryan and Texas senator Ted Cruz, one of the 16 candidates who lost to Trump.

The convention logo shows a trumpeting elephant, the icon of the Republican Party, caught in a charge across an electric guitar, a nod to Cleveland’s Rock and Roll Hall of Fame and Museum – fitting symbols for the Trump Show next week.

"It is likely to be the most unconventional convention in modern times," says Whit Ayres, a Republican pollster, who backed the Florida senator Marco Rubio in the Republican race.

“Trump has run an unconventional campaign where he is running no ads in swing states against the Democratic ad machine. He has said he doesn’t need to unify the party despite the fact that every winning Republican candidate has won 90 per cent of the Republican vote. He is convinced he has redefined the modern Republican campaign. We will see if he is right or not.”

The victory of Trump the outsider and the fissures his candidacy has created within the party also make this year's convention unusual. Not since 1976, and the fight between Gerald Ford and Ronald Reagan, has there been so much uncertainty and division before a Republican convention. Anti-Trump forces like to point out that 55 per cent of Republican voters backed someone else in the primaries and that he has an even higher negative approval rating.

"It can be the Trump Show as long as voters recognise that he is not representing Republicans," says Fergus Cullen, a former chairman of the New Hampshire Republican Party and a diehard Never Trumper who backed Ohio's Republican governor, John Kasich.

The 2016 convention is different for another reason. None of the last three Republican nominees is attending next week's convention. The 2012 nominee, Mitt Romney, a staunch anti-Trumper, is, along with the last two Republican presidents, George HW Bush and George W Bush, skipping the biggest event in the party's four-year presidential cycle.

"That really is unusual, and what it really signifies is that Trump is a hostile takeover of the party," says Paul Beck, professor emeritus of political science at Ohio State University. "Most conventions will trot out the major figures within the party even if those major figures are retired from elected life. It is all designed to build support for the general-election campaign. You are not going to see much of that in Cleveland."

John McCain, the 2008 nominee, who lost to Barack Obama, is avoiding the convention, along with other elected Republicans, such as Senators Kelly Ayotte of New Hampshire and Mark Kirk of Illinois, who are fighting for re-election down the ticket from Trump in November. Trump is toxic, particularly in states such as Arizona, which has a large Hispanic population that is alienated by Trump's anti-Mexican comments.

Trump’s rival Hillary Clinton points out the disunity within the Republican Party over his candidacy on a near-daily basis. Other Democrats revel in the disarray.

"They might bring Clint Eastwood back – or at least the chair," says the Democratic strategist Brad Bannon, referring to the much-ridiculed speech the Hollywood star delivered to an empty chair on the stage of Romney's 2012 Republican convention. "Or maybe they could bring in the first two wives if they're really stuck for speakers."

While the power of celebrity raises attention, conventions traditionally have two more mundane practical purposes: to unify the party and introduce the presidential ticket to millions of viewers. Trump seems intent on ticking only one of those boxes. Although he has said that he wants to use the convention to build party unity, he has also said that he may not need it. “I have to be honest: I think I’ll win without the unity,” he said last week.

"He is not really trying to unite the party. He just expects everybody to support him, and that is just not going to happen given the type of primary campaign he ran," says Ryan Williams, the former deputy national press secretary for Romney's 2012 presidential bid.

“It is a major problem for him that he doesn’t have the support of Mitt Romney, of both President Bushes and other party elders. Tens of millions of people voted for those candidates. If you are going to be president you need every single vote.”

Kasich, the last Republican to concede defeat to Trump in the presidential race, and who has refused to endorse him, has said he is going to skip a convention he worked so hard to bring to his home state. It is highly unusual for a host governor not to attend his or her party convention.

The two-time governor, once hopeful of disrupting Trump’s path to the 1,237 votes to secure the nomination, has repeatedly said that convention delegates awarded to Trump in the primaries should “weigh their responsibilities against their consciences” before deciding whether to support him.

Despite the efforts of the anti-Trumpers, the odds of blocking his path to the nomination at this point remain long given the level of support he won in the primaries. In a big win for Trump, a Republican rules-committee meeting in Cleveland on Thursday night rejected a motion from the Never Trump movement to allow delegates to vote with their consciences.

“The Never Trump forces are playing for second place, which is just to gum up the works and kind of embarrass him a bit instead of denying the nomination,” says Williams.

Dussault, a neutral, thinks Trump is not a viable candidate for the most powerful office on earth, but he believes Republicans have spoken and that his victory reflects democracy at work. “The people voted for him and put him in that position,” he says. “If something strange happens, and they don’t allow him to follow through, there are going to be problems.”

In that scenario Trump’s prediction may prove accurate.