Saturday night's GOP debate was the highest-rated of the election cycle since December - smashing suggestions voters are growing weary of the format.

Around 13.5 million people tuned in to CBS's broadcast at 9pm to see Donald Trump's messy clash with Ted Cruz as the field of candidates narrows.

The viewer figures released on Sunday signal a steady climb in interest in the Republican stand-offs - though still short of the 15 million who tuned in to the Las Vegas debate in early December.

The first debate on Fox Business in January drew 11 million. Two weeks later, 12.5 million tuned in to the Fox GOP debate. Last Saturday, 13.3 million watched ABC's.

Meanwhile Democrats Hillary Clinton and Bernie Sanders have failed to garner the same level of interest.

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The Democratic National Committee gleefully passed around tweets from Republicans mocking their own party's candidates

The verdict was swift and near universal on Saturday night: the GOP-sponsored presidential debate was such a 'train wreck' that even some Republicans were burying their heads in their hands

They drew a total of just eight million to their latest head-to-head, broadcast on both CNN and PBS this Thursday.

Indeed, the debate on Saturday was not short of action.

Ted Cruz and Marco Rubio - both of Cuban heritage - engaged in a bitter back-and-forth about their grasp of the Spanish language.

Once again, they traded slights over immigration. This time, however, Cruz claimed the Florida senator went on Univision in Spanish and said he would not rescind President Obama's illegal executive amnesty on his first day in office.

Rubio hit back: 'I don't know how he knows what I said on Univision because he doesn't speak Spanish.'

Cruz barked back at him in Spanish to prove that he does indeed speak his ancestors' native language.

Trump got booed by the audience when he went off about 9/11 and the war in Iraq.

The Donald continued to contend that the audience was stacked against him.

The Saturday night debate saw Donald Trump and Jeb Bush get into multiple scuffles, including when the New York billionaire attacked former Republican president George W. Bush for allowing 9/11 to happen on his watch

Republicans were astounded by the viciousness. Democrats could barely contain their delight.

Correct the Record, a super PAC backing Hillary Clinton, sent reporters a video of the evening's highlights - or lowlights, depending on the perspective - that included a graphic of two boxing gloves and the text 'GOP Debate: It's a brawl'.

The Democratic National Committee gleefully passed around tweets from Republicans mocking their own party's candidates.

'Seriously, this is insane. The GOP is destroying itself tonight, and they have no one to blame but themselves,' Republican pollster Frank Luntz tweeted.

Rich Lowry, the editor of the conservative National Review, said, 'Train wreck of a debate, no?'

The former national spokeswoman for Rand Paul's presidential campaign, Eleanor May, said, 'Real Housewives of the #GOPDebate. So many cat fights, no real solutions.'

Robert Moran, a partner at Brunswick Group, an advisory firm with a specialty in corporate relations said: 'I have been a Republican all of my adult life. This was a trainwreck. There are only 3 candidates that could lead.'

Taking the most delight in the six-way shouting fest, perhaps, was leading Democratic presidential candidate Hillary Clinton, whose name came up just eight times as the Republican candidates cannibalized each other

Clinton was one of many who criticized the debate, which was called a 'train wreck' even by Republicans

Another self-proclaimed Republican on Twitter named Ryan said, 'I'm 40 minutes into this train wreck, I mean #GOPDebate-- What the f***? Trump needs tranquilized and put in a straight jacket... Lunatic.'

The Atlantic's David Frum chastised attendees of the fight night as he said, 'And the audience is joining in the bloodbath. Does this look to America like a party ready to govern anything?'

Besides booing Trump for his comments on the Bush legacy, the audience also challenged debate moderator Dickerson for fact-checking something Cruz suggested on the Senate's record of approving Supreme Court nominees directly before a presidential election.