Republican presidential candidates Donald Trump (left) and Sen. Ted Cruz, R-Texas, participate in Thursday's GOP debate, Jan. 14, 2016, in North Charleston, South Carolina. Scott Olson/Getty Images

Real estate mogul Donald Trump sparred with Ted Cruz at the sixth Republican primary debate Thursday night, confirming pundits' suspicions that he began attacking Cruz on his citizenship because the U.S. senator from Texas is climbing in the polls. In recent days, Trump suggested Cruz can't run for president because the Constitution requires all candidates to be "natural born" citizens. The precise definition of that phrase hasn't come up in federal court, but Cruz was born in Canada to an American mother.

Trump seized upon the gray area Thursday. "We're running, I choose him as my vice presidential candidate, and the Democrats sue because we can't take him along for the ride. I don't like that, OK?" Trump said. "If you become the nominee, who the hell knows if you can ever serve in office."

When asked why he's taking on Cruz, Trump — who's focused his mudslinging thus far on candidates like former Florida Gov. Jeb Bush — was honest. "Because now he's doing a little better," Trump admitted.

Undeterred, Cruz quickly fired back. "I'm not going to be taking legal advice from Donald Trump," he said.

Heading into Thursday's debate, Trump was keeping a firm hold on his No. 1 spot. He had the support of 37.8 percent of likely Republican primary voters, according to the HuffPost Pollster, beating out Cruz at 19 and Florida Sen. Marco Rubio at 10.4 percent.

Cruz has previously said Trump's only bringing up the citizenship debate because he is "a little bit rattled." Cruz went on to criticize Trump's New York values and political inexperience, but Trump remained on the defensive going into the debate, according to the Washington Post.

“They attack,” Trump said, “but they don’t understand that unlike this country, I attack back.”

Political adviser Eric Fehrnstrom told Reuters all the drama would make for an exciting show Thursday. "Given all the attacks that are taking place and the counter attacks, I think it will be a more lively debate than we’ve seen at this point," Fehrnstrom said. "Donald Trump is still in complete command and the race is still about who will become his main challenger."

The next GOP debate was scheduled for Jan. 28, just days before the Feb. 1 Iowa caucuses.