Donald Trump’s battle with the GOP ‘establishment’ hit new levels of intensity hours before Thursday’s Fox News debate in Detroit, as Mitt Romney tried to rally Republican leaders and voters against the billionaire businessman’s front-running campaign – and Trump dismissed his complaints as “irrelevant.”

Trump’s feud with the 2012 Republican nominee sets the tone ahead of the primary debate in Detroit Thursday night, when he will face off against a narrowing field of rivals on stage.

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Retired neurosurgeon Ben Carson will not attend, after announcing a day earlier he sees no path to the nomination following his Super Tuesday losses. The remaining candidates on stage are Texas Sen. Ted Cruz, Florida Sen. Marco Rubio and Ohio Gov. John Kasich.

Romney’s searing address earlier Thursday in Utah denouncing Trump’s campaign – in which he called Trump a “phony” who would sink the country into recession -- is likely to fuel arguments by the other three candidates that Trump is not presidential material.

Kasich, for his part, applauded Romney for his address. And Rubio, who unloaded on Trump at their most recent debate, has shown little interest in dialing back, especially as the window for challenging the front-runner narrows.

But Trump, who rarely lets an attack go unanswered, was unsparing in his response to Romney.

Trump answered back by calling Romney a “choke artist” and a “failed candidate.” As he swatted away Romney’s criticism, he contrasted the 2012 presidential nominee’s failed bid against his own campaign.

“This is a movement,” Trump said at a rally in Maine, en route to Detroit.

Trump increasingly finds himself positioned between two powerful forces – the primary voters who are fueling his surging campaign, and members of the GOP establishment trying to stop him.

Indeed, Super Tuesday showed primary voters once again breaking in huge numbers for Trump’s campaign – delivering him seven state victories out of 11 and building his already-substantial delegate lead over his rivals.

Trump’s success on Tuesday sparked the latest round of hand-wringing by Republican groups and figures, weighing whether to get on board the Trump train or do whatever they can to derail it.

Each of the remaining presidential candidates is continuing to campaign, undeterred by Trump’s dominance on Super Tuesday and his delegate lead.

Cruz won three states, including his home state of Texas, on Tuesday. Rubio won his first, in Minnesota. Kasich has not won any, but is banking on his home state’s primary on March 15.

Cruz has openly applied pressure on the two other candidates to consider dropping out, arguing that he is the only candidate who can take on Trump in the remaining primary contests.

Rubio, though, has enjoyed substantial support from fellow GOP lawmakers and is banking on a comeback in his home state on March 15 as well.

Kasich, speaking to reporters in Detroit, vowed again he would win Ohio and predicted that if he does, the race will probably come down to the convention.

“That’s what it looks like,” he said.