The Kentucky Republican hit his colleague and fellow presidential candidate on Fox News on Monday for being born in Canada and perhaps not fitting the constitutional requirement that a President be a natural-born citizen.
"Cruz is a natural-born Canadian," Paul said. "He was naturally born there. The question is, can you be natural-born Canadian and natural-born American at the same time? Maybe, but I think the courts will have to decide it, because it's never really been decided."
Paul was echoing attacks started by GOP front-runner Donald Trump, who has attacked the Texas senator over his citizenship status as Cruz has drawn closer to the billionaire businessman in the polls and overtaken him in Iowa.
But Paul, like Trump, insisted he wasn't raising the issue and that it's the Democrats who will make it a problem if Cruz is the nominee.
"Democrats have already threatened. There is one Democratic congressman who said he will file a lawsuit over this," Paul said, referring to Rep. Alan Grayson, D-Florida. "It is not me, it's whether or not Democrats pursue it -- and I think they will."
Legal experts have said Cruz is a natural-born citizen because his mother was an American citizen when he was born in Canada, but the issue has never been settled by the federal courts.
Paul said though it's clear Cruz has a right to citizenship, it's not clear if it's the same as what the Constitution requires of a President.
"For traditional citizenship it always has been (your parent), but the only part of the Constitution that says 'natural-born' is with regard to the President. So it appears to be a unique qualification and most people have interpreted they had to be born in the U.S. until recently," Paul said. "So I think eventually the Supreme Court will probably have to decide it."
Republicans have split on the Cruz controversy. Some of his competitors have side with Trump that it is a liability, while others, including former presidential candidate Mitt Romney, have said there's no question Cruz is eligible.
Paul is lagging in the polls and was also speaking to Fox News about being at risk of not making the main stage at the next presidential debate on Thursday.
Cruz has laughed off the dust-up.
"I recognize that there are candidates in the field that don't want to talk about those issues, and they want to instead encourage the good people of the media to go down rabbit trails and engage in silly sideshows," Cruz said. "I don't think the American people are interested."