Donald Trump speaks as Wisconsin Gov. Scott Walker listens Thursday night at the Quicken Loans Arena in Cleveland. The Associated Press

Republican presidential candidates, from left, Chris Christie, Marco Rubio, Ben Carson, Scott Walker, Donald Trump, Jeb Bush, Mike Huckabee, Ted Cruz, Rand Pau, and John Kasich take the stage for the first Republican presidential debate, on Thursday night at the Quicken Loans Arena in Cleveland. The Associated Press

Sen. Rand Paul, R-Ky., speaks during the debate. Paul challenged Donald Trump's statement that he will consider running a third-party campaign if he doesn't win the Republican nomination. The Associated Press

Former Florida Gov. Jeb Bush speaks during Thursday's debate in Cleveland.

Sen. Marco Rubio, R-Fla., speaks during Thursday's debate. The Associated Press

New Jersey Gov. Chris Christie speaks as Sen. Marco Rubio, R-Fla., listens during the debate. The Associated Press

Donald Trump responds to former Arkansas Gov. Mike Huckabee's closing remarks. Trump lived up to his sharp-edged reputation during Thursday's debate. The Associated Press

Republican presidential candidate Carly Fiorina, second from right, is joined on stage from left by Bobby Jindal, Rick Perry, Rick Santorum, and George Pataki during Thursday's pre-debate forum. Seven of the candidates did not qualified for Fox News' prime-time debate. The Associated Press