The latest round of Republican presidential debates Thursday night featured the first outright clash between frontrunners Donald Trump and Senator Ted Cruz (R-TX), as the two traded blows after having largely refrained from direct confrontations in the race until now.

In the debate on Fox News, the tensions between the two were palpable as the polls in Iowa show them running close ahead of the February 1 caucuses. Some polls have shown Cruz beating Trump in Iowa.

Trump took aim at Cruz, saying he might not be eligible to run for office because he was born in Canada, even though Cruz’s parents were both American citizens. When asked why he was raising the issue now, Trump admitted that he’s now going after the senator because he’s gaining in the polls.

Cruz rebuffed the aspersions on his eligibility, noting, “back in September, my friend Donald said he had his lawyers look at this in every which way. There was nothing to this birther issue.”

“Since September, the Constitution hasn’t changed. But the poll numbers have,” quipped Cruz, to the laughter of the audience.

The Texas senator also noted that some more extreme critics say that both parents have to be born in the US in order to be eligible, and pointed out that by those standards Trump would himself be ineligible as well, given that his mother was born in Scotland.

“On the issue of citizenship, I’m not going to use your mother’s birth against you,” said Cruz, to which Trump responded, “but I was born here…big difference.”

Trump also half-jokingly raised the idea of naming Cruz as his vice president, to which Cruz replied with the same offer. However, the real estate mogul said he’d probably “go back to building buildings” if he failed to get the nod.

Later in the debate, Cruz criticized Trump for his “New York values,” intimating that he has a liberal mindset, at which point Trump cited the September 11, 2001 attacks to close the issue.