One question has tripped up Republican candidates more than any other: Who will be toughest on illegal immigration?
Donald Trump has claimed Sen. Ted Cruz of Texas is for "amnesty" -- and vice versa. Both have criticized Sen. Marco Rubio of Florida, who has gone after Cruz.
Jeb Bush piled on Thursday, saying Rubio "cut and run" after leading the Senate's "Gang of Eight" immigration bill "because it wasn't popular."
"You used to support a path to citizenship," Rubio snapped.
"So did you," Bush shot back.
All the GOP candidates promise to end illegal immigration, but what to do with those here illegally?
Trump says that the estimated 11 million here illegally "have to go" and that he would set up a special federal force to accomplish the task, while Cruz says he's not going "to send jackboots to knock on your door and every door in America."
Rubio said Thursday he's not going to "round up 12 million" immigrants.
Cruz continued to be attacked over an amendment he offered in a Senate committee to the Gang of Eight immigration reform bill, which Rubio co-sponsored. Cruz's proposal would have boosted immigration.
Cruz's amendment was never expected to pass the committee. He now calls it a "poison pill" designed to derail the broader bill, while Rubio insists it was a real proposal.
"This is the lie that Ted's campaign is built on -- that he's the most conservative guy," Rubio said at Thursday's debate. "The truth is, Ted, throughout this campaign, you've been willing to say or do anything to get votes."
"I like Marco, he's very charming, very smooth," Cruz responded. "Marco made the choice to go the direction of the major donors to support amnesty because he thought it was politically advantageous."
Actually, it was a little of both. Cruz in 2013 was having it both ways: He could claim to be for immigration reform -- as the GOP was more supportive of the issue at the time -- but vote against the bill because his idea wasn't included.
Cruz now says Trump is the one backing "amnesty," because Trump wants to let the "really good people" who are currently in the country without legal authorization return to the U.S. after they are deported.
"People that have come into our country illegally, they have to go," Trump said during the GOP debate in Las Vegas in December. "They have to come back in through a legal process."
That's often referred to as a touch-back provision, which would require those here illegally to return to their home country and apply again to reenter.
Cruz calls it "amnesty."