Transcript for Donald Trump Calls Hillary Clinton 'a Liar'
That accusation that Donald Trump's words are helping Isis recruit terrorists set off the republican front runner today. Here's Devin Dwyer. Reporter: At the democrats' debate, only one republican candidate was mentioned by name. Donald Trump. Donald Trump. Reporter: This morning on ABC's "This week," Donald Trump fired back, taking on Hillary Clinton's explosive allegation that trump is showing up in Isis recruitment videos. Knowing the Clintons and knowing Hillary, but there's no -- there's nobody -- she just made it up. I mean, she made it up. It was a sound bite. Reporter: After the Clinton campaign couldn't provide hard evidence of videos, a top Clinton adviser, pressed by George stephanopoulos, tried to clarify. She didn't have a particular video in mind, but he's being used in social media. Reporter: It wasn't the only Clinton moment last night that made waves. We now finally are where we need to be. We have a strategy and a commitment to go after Isis. Reporter: Jeb bush overnight turning her comment into a campaign attack video. America has had enough of empty words of declarations detached from reality. Reporter: As the struggling candidate also lashed out at the gop frontrunner. I have to get this off my chest. Donald Trump is a jerk. Reporter: Trump seems happy to have friends elsewhere. If Putin wants to call me brilliant and other things that he said which were, frankly, very nice, I'll accept that. Reporter: Meanwhile, his rivals Marco Rubio and Ted Cruz are now in an all-out brawl over immigration. I oppose amnesty. I oppose citizenship. I oppose legalization. Period. The end. The fact of the matter is that on multiple issues, he's changed his position multiple times. Reporter: It's a battle for second place nationally as trump heads into the holidays with a solid lead. The next debate, mid-january. Tom? Thank you. We turn next to the scare onboard a flight to Paris.
This transcript has been automatically generated and may not be 100% accurate.