The burden is particularly heavy on U.S. Senator Marco Rubio of Florida and Ohio Governor John Kasich, who must do well in their home states or face pressure to exit the race.
While Kasich is holding his own against Trump in Ohio opinion polls, Rubio has lagged far behind Trump in Florida.
Trump’s victories in Michigan, Mississippi and Hawaii on Tuesday added to his momentum after a week of attacks by mainstream Republicans who are offended by his statements on Muslims, women and illegal immigrants and alarmed by his threats to international trade deals.
The tough-talking billionaire businessman told CNN on Wednesday he expected the debate to be a «nicer, softer, lighter debate, I hope.» But he added: «I’ll be ready. I’m the only one who can beat Hillary,» a reference to Democratic presidential front-runner Hillary Clinton.
The tone could be set by Trump, 69, who forecast before several previous debates that he would take a more measured approach, only to quickly get into a verbal brawl with his rivals.
The CNN-hosted debate will be held at the University of Miami at 8:30 p.m. (0130 GMT on Friday).
«I would focus on a positive uplifting message that takes us back to why they’re running for president and what’s at stake in this election,» said Eric Fehrnstrom, a senior adviser to 2012 Republican nominee Mitt Romney.
In Detroit last Thursday, Trump came under sustained fire from Rubio, 44, over parts of his business empire such as Trump University, a now-defunct venture that critics said offered a flimsy education. U.S. Senator Ted Cruz of Texas piled on Trump over his past support for Democratic causes.
Trump blasted Rubio throughout the debate as a lightweight and Cruz as a liar.
The Trump-Rubio debate clash in Detroit followed an abrupt change in Rubio’s campaign to begin raising personal questions about Trump, such as saying he had small hands, a statement Trump saw as a reference to his penis size.
Rubio said he now regretted the personal insults.
«In terms of things that have to do with personal stuff, yeah, at the end of the day, it’s not something I’m entirely proud of. My kids were embarrassed by it, and if I had to do it again, I wouldn’t,» Rubio told MSNBC on Wednesday.
Cruz, 45, who would like to knock Rubio and Kasich out of the race so he could draw more support from anti-Trump Republican voters, will seek to use the debate to position himself as the best Trump alternative.
«Trump is clearly trying to reach out and be a little more statesmanlike,» said Cruz backer Saul Anuzis, a former chairman of the Michigan Republican Party. «Cruz is going to make the case as to why he can beat Trump and be the viable alternative.
From our perspective, we think it’s a two-man race, we’re the only one with a path.»
Rubio is fighting for survival and is facing something of a last stand in Florida. Rival campaigns said Rubio may feel a need to return to the optimistic message he stuck to for months before getting sidetracked by Trump.
Rubio told Fox News on Wednesday the campaign was always going to come down to who wins Florida. «The nominee has to win Florida. You can’t be the Republican nominee if you don’t win Florida,» he said.
Kasich, Rubio and Cruz have one shot at denying Trump the nomination – preventing him from assembling the required 1,237 delegates to win the nomination outright, which would mean delegates would have to choose the nominee at their July 18-21 convention in Cleveland.
Trump said on CNN on Wednesday that 1,237 was an «artificial number» and that if he entered the convention with more delegates than his rivals, he should be the nominee.
While he has vowed to wage war against the party if he is denied the nomination at the convention, Kasich, 63, said it was possible for Republicans to have a contested convention and emerge united.
«Can you go to a convention and come out united? Absolutely,» he told a Fox News town hall on Wednesday.