A smaller cast of candidates faces off Tuesday night in the Republicans’ fourth presidential debate, with mild-mannered Ben Carson pledging to push back aggressively if challenged on the veracity of his celebrated personal story. Florida Sen. Marco Rubio is expecting more confrontation, too, from rivals trying to block his momentum.

The debate, the last for the GOP until mid-December, could help shape the course of the campaign into the winter as voters begin to pay more attention to the White House race.

Billionaire Donald Trump has led the field for months, defying standard political logic, while experienced governors and senators have struggled to break through. Another outsider, Carson, the quiet retired neurosurgeon, began challenging Trump’s grip in recent weeks. As he’s risen in preference polls, however, Carson has faced a flurry of questions about his biography, which has been central to his connection with voters.

His campaign manager, Barry Bennett, said Carson is prepared to be far more aggressive in the prime-time debate and is “a lot more fired up” after facing days of questions about his past.

“He’s not going to attack anybody,” Bennett said. “But if somebody goes after him, they’re going to see a lot more ‘back at ’em’ than they ever saw before.”

While pieces of Carson’s background had been challenged earlier in the campaign, the questions ballooned last week after CNN reported it could not find friends or confidants to corroborate the story, told in his widely read autobiography, of his unsuccessfully trying to stab a close friend when he was a teenager.

Later in the week, Politico examined Carson’s claim of having been offered a scholarship to attend the U.S. Military Academy, and The Wall Street Journal said it could not confirm anecdotes told by Carson about his high school and college years.

In a GOP primary where bashing the media is in vogue, Carson could come out ahead if the moderators of Tuesday’s debate on Fox Business Network are seen as unfairly piling on. Carson’s campaign was active in the effort to change how the party’s debates are run after several candidates expressed unhappiness with moderators from CNBC at an event two weeks ago.

Yet some Republicans say Carson must walk a fine line.

“Will viewers and voters see the unflappable surgeon they have been inclined to support or will a more combative Carson emerge?” said Matt Strawn, the former chairman of the Iowa Republican Party. “If the latter, his standing may well suffer if he appears to be yet another politician trying to out-outrage the others on stage.”

Trump previewed a potential line of attack at a rally in Springfield, Illinois, on the eve of the debate.

“With what’s going on with this election, I’ve never seen anything like it. People are getting away with murder,” Trump said. “If you try and hit your mother over the head with a hammer, your poll numbers go up. I never saw anything like it!”

Rubio will walk onto the stage with some momentum following a strong performance in the most recent debate. The senator is widely seen as among the most talented politicians in the GOP field, and his rise appears to have worried some of his rivals.

That’s especially true of Jeb Bush, the former Florida governor who is a friend and political mentor for Rubio. Bush struggled in his attempts to challenge Rubio in the last debate, though he’s continued to needle him in campaign appearances, and his well-funded super PAC has signaled plans to aggressively go after the senator in the coming weeks.

Trump has also stepped up his criticism of Rubio, calling him a “total lightweight” and a “highly overrated politician” in tweets sent late Monday night.

Rubio’s campaign tried to get ahead of some expected challenges in the debate by releasing two more years of charge card statements from his time as a state lawmaker. He has faced questions about his use of an American Express card issued by the Republican Party of Florida for some personal spending.

Also in the main debate Tuesday night is Ted Cruz, who is enjoying new momentum following a strong performance in the last contest, and businesswoman Carly Fiorina and Ohio Gov. John Kasich.

Missing from the lineup are New Jersey Gov. Chris Christie and former Arkansas Gov. Mike Huckabee. Both were dropped from the top-tier debate with low poll numbers in national surveys, sparking criticism for the way networks hosting the debates have determined participation.

Christie and Huckabee will instead appear in an undercard debate, along with Louisiana Gov. Bobby Jindal and former Pennsylvania Sen. Rick Santorum.

“We’re not whiners and moaners and complainers in the Christie campaign,” Christie said on Fox News. “Give me a podium, give me a stage, put the camera on, we’ll be just fine.”

(AP)