"One of them is a socialist; America doesn't want to be a socialist country," Rubio said. "The other one is under FBI investigation and not only is she under FBI investigation, she lied to the families of the victims of Benghazi."
Cruz said he would back Trump "because I gave my word that I would, and what I have endeavored to do every day in the Senate is do what I say I will do.”
Kasich responded: "If he ends up as the nominee — sometimes he makes it a little bit hard — but I will support whoever is the nominee for president.”
And Trump reaffirmed his pledge to support the eventual nominee even if he does not win the party's nomination as the establishment continues its public wrestling with whether to support him.
"Even if it's not me?" he asked, before responding, "Yes, I will," to moderator Chris Wallace.
He went on to claim that "millions and millions" of people have joined the Republican Party because of him, giving some credit to his rivals.
The scene was reminiscent of the first question in Fox News's first debate back in August, but during that time, Trump said that he wouldn't swear off a third-party bid.
A few months later, Trump publicly signed a pledge not to make that insurgent bid in a move that quelled concerns from the party about a shake-up that could siphon off votes from the party's nominee.
But since then, Trump has publicly waffled on that pledge, tweeting about polls that mention that his supporters would go with him for a third-party bid.
A handful of GOP lawmakers have declared that they would not support Trump, while party operatives are mounting a last-ditch effort to blitz the airwaves with anti-Trump messaging.
And Mitt Romney bashed Trump on Thursday with a full-throated rebuke of the GOP front-runner.