NORTH CHARLESTON, S.C. (AP) — 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 roughand 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.