The fifth Republican presidential debate began with questions about Donald Trump's proposed ban on most Muslims from entering the U.S., with Jeb Bush firing back at the real estate mogul's controversial plan.
"Donald is great at the one-liner, but he's a chaos candidate. And he'd be a chaos president," Bush said.
Trump responded by bashing the former Florida governor's stalled campaign.
"Jeb doesn't really believe I'm unhinged. He said that because he has failed in this campaign," Trump said. "It's been a total disaster."
Trump's newest rival atop the polls, Ted Cruz, declined to follow Bush in critiquing Trump, as did Marco Rubio.
Cruz said he "understands" why Trump has called for a ban on Muslims from entering the country, but that not all are seeking to do the U.S. harm.
The Bush-Trump back-and-forth continued throughout the debate, with Bush intent on taking on Trump on issues of national security.
"I won't get my information from the shows. I don't know if that's Saturday morning or Sunday morning," Bush said, a reference to Trump saying he gets foreign policy advice from television.
Bush continued to act as an attack dog against Trump while his higher polling rivals largely stayed out of the fray. At one point Trump began to complain that the questions fed to Bush were set up to bash him.
"You think this is tough and you're not being treated fairly? Imagine what it's going to be like dealing with Putin," Bush quipped.
"Oh I know, you're a tough guy Jeb," Trump countered.
This article originally appeared on NBCNews.com.