It's already being called Las Vegas Fight Night -- the moment the Republican presidential rivals turned the heat on each other, and the moment it all got personal.

In a desperate bid to knock frontrunner Mitt Romney off his seemingly inevitable course, the rest of the pack piled into him during last night's GOP debate, attacking his stance on everything, from immigration to healthcare.

The man most desperate to restore some glitter to his rapidly fading star, Rick Perry, could barely contain his hostility. There were narrowed eyes. There was shouting. There was even some actual snarling.

It all came to a head over an episode in 2007 when Romney hired a lawn company which was later found to have employed illegal immigrants. Not so much a debate, more of a slanging match ensued, as Perry went in for the kill (see video above). "The idea that you stand here before us and talk about that you're strong on immigration is, on its face, the height of hypocrisy." Romney fired back, with a sort of hollow laugh: "Rick, I don't think I've ever hired an illegal in my life. And so I'm looking forward to finding your facts on that." That made Perry even madder: "It's time for you to tell the truth".

Outside, chief strategists for the two men were frantically trying to spin this loss of cool -- Perry's man accusing Romney of being "very rattled" -- while the Romney lot called Perry a desperate hothead.

And we haven't even got around to Herman Cain yet. The man who's been briefly enjoying his moment as the anyone-but-Romney favourite had an even worse night.

In the debate itself, he came under repeated questioning over his 9-9-9 tax plan, which wasn't helped by a report from the non-partisan Tax Policy Centre. They said his proposals would cut taxes for 95 per cent of America's millionaires, while raising them for most people earning less than $100,000. And the six other candidates siezed their chance, dubbing it dangerous and simplistic.

Asked about foreign policy, the former pizza mogul was stumbling all over the place. At one stage, he said he could see himself negotiating with al-Qaeda, if he were elected. He later had to back down on the Anderson Cooper show, admitting he had "misspoke", before blundering on: "Because I didn't, you know, things are moving so fast, I misspoke."

Turns out he'd got himself all confused between the whole al-Qaeda thing and Israel's decision to release hundreds of prisoners in exchange for the captured soldier Gilad Shalit. Which doesn't exactly inspire confidence in a man vying to be the figure with his finger on the nuclear button.

This, after FEC returns showed Cain spent $36,000 of his campaign funds buying copies of his own book, perhaps in an effort to keep it in the New York Times bestseller list.

Last night, though, was really all about Romney, or stopping Romney. As Politico put it, for his rivals "it's a strategic imperative to halt the frontrunner". Yet all the acrimony, the shouting matches, and the attacks on each other may not have impressed any wavering voters who tuned in. And one man -- President Obama -- got off relatively unscathed. He is deep in traditional Republican territory, trying to drum up support in North Carolina and Virginia.

On that note, perhaps the last word should go to one of those warring Republicans, Newt Gingrich, who warned that "maximising bickering" might not be the best way to win the White House.

Felicity Spector is a senior producer at Channel 4 News