NORTH CHARLESTON, S.C. — With just over two weeks until voting begins, Republican presidential candidates Donald Trump and Ted Cruz firmly asserted their standing atop the GOP race in a fiery debate, overshadowing a crowded field of rivals still grappling for a way to overtake the front-runners.

Thursday night’s debate underscored that the competition between Trump and Cruz will be rough-and-tumble in the days leading up to the Feb. 1 Iowa caucuses, a shift from the relative civility that’s defined their relationship until now. The candidates tangled over Cruz’s eligibility to serve as commander in chief and the real estate mogul’s “New York values,” with Trump besting his rival with an emotional recounting of his hometown’s response to the Sept. 11 attacks.

“When the World Trade Center came down, I saw something that no place on Earth could have handled more beautifully, more humanely than New York,” Trump said. “That was a very insulting statement that Ted made.”

Trump renewed his suggestion that Cruz may not be eligible to serve as commander in chief, saying the senator has a “big question mark” hanging over his candidacy, given his birth in Canada to an American mother. Cruz suggested Trump was only turning on him because he’s challenging for the lead in Iowa — and the businessman agreed.

Thursday’s debate was one of the last high-profile opportunities other candidates on stage had to sway voters’ views. But none appeared to emerge with a breakout moment.

Florida Sen. Marco Rubio, who holds a slight advantage over the field of more mainstream candidates, found himself in heated exchanges with both Cruz and New Jersey Gov. Chris Christie.

Rubio likened Christie’s policies to President Barack Obama’s, particularly on guns, Planned Parenthood and education reform — an attack Christie declared false. Seeking to undermine Rubio’s qualifications for president, Christie suggested that senators “talk and talk and talk” while governors such as himself are “held accountable for everything you do.”

Cruz confronted Rubio late in the debate over his support for a Senate bill that would have created a pathway to citizenship for people in the U.S. illegally, an unpopular position among GOP primary voters. Rubio tried to flip the criticism around on Cruz, accusing him of switching positions on immigration himself, as well as on numerous other issues.

“That is not consistent conservatism,” Rubio said. “That is political calculation.”