Sen. John McCain questioned whether Sen. Ted Cruz (R-Tex.), who was born in Canada, is eligible to be president.

McCain, who has long clashed with Cruz in the Senate, said on KFYI Wednesday that "it's worth looking into" whether Cruz is a natural born citizen, a requirement to be president. The assertion comes the day after Donald Trump, whom Cruz is leading in polls in Iowa, told The Washington Post that Cruz's birthplace could be "very precarious" for the GOP.

When asked how Cruz could run for president if he was born in Canada, McCain answered, "I do not know the answer to that."

McCain was born in the Panama Canal Zone in 1936, and the Senate unanimously passed a resolution declaring him a natural born citizen when he ran for president in 2008. McCain said Wednesday the issue is different because the Canal Zone was a territory and U.S. Military base and there was precedent set when Barry Goldwater, who was born in Arizona when it was a territory, ran for president.

"That's different than being born on foreign soil," he said.

"I think there is a question," he said of Cruz. "I am not a constitutional scholar on that but I think it's worth looking into. I don't think it's illegitimate to look into it."

WATCH: Donald Trump warms up to Ted Cruz after months of his compliments on the campaign trail. (The Washington Post)

Cruz was born in Calgary, Alberta, in 1970 to an American citizen mother and Cuban-born father. He has said he is an American citizen by virtue of his mother's U.S. citizenship. He has renounced his Canadian citizenship. Legal scholars have said Cruz meets the requirement of natural born citizenship, though it is untested in the courts.

Sen. Rand Paul, who is also seeking the Republican nomination, also brought up the issue Wednesday, stating Cruz is eligible to be prime minister of Canada.

Cruz initially responded to Trump's comment by tweeting a clip of an episode of "Happy Days" where a character water skis over a shark, which has come to mean something is overdone. Speaking to reporters, he said that the public wants to focus on substantive issues. The media, he said, "loves to ... gaze at their navels for hours on end by a tweet from Donald Trump or from me or from anybody else. Who cares?"

On Wednesday Cruz again asserted that the media want him and Trump to fight -- and that he doesn't intend to do so. Cruz's campaign did not immediately respond to McCain's comments.