First, it was President Obama. Lately, it's been Ted Cruz. Now, it's Marco Rubio whose White House eligibility isn't a sure thing, according to Donald Trump.

On Saturday, the day of the South Carolina Republican primary, Trump retweeted a message asseerting that both of his top rivals for the GOP nomination aren't even eligible to be president.

Then, in an interview on ABC's "This Week" on Sunday, Trump refused to concede Rubio's eligibility when pressed by host George Stephanopoulos.

STEPHANOPOULOS: You're really not sure that Marco Rubio is eligible to run for president? You're really not sure?

TRUMP: I don't know. I've never looked at it, George. Honestly, I've never looked at it. Somebody said he's not, and I retweeted it.

Questions about Cruz's eligibility have been treated as legitimate by constitutional scholars and news media because the Texas senator was born in Canada. The constitution mandates that a president be a "natural-born citizen" but does not explicitly define the term.

The prevailing opinion is that "natural-born" means a citizen at birth — which Cruz was, by virtue of being born abroad to a U.S. citizen (his mother). Previous Republican presidential nominees John McCain and Barry Goldwater also were born outside the country. However, some scholars maintain that only a person born within U.S. borders is "natural-born." By that standard, Cruz would be ineligible.

Rubio satisfies both definitions of "natural-born." He was born in Miami and was therefore a citizen from the moment he left the womb.

The basis of any skepticism about Rubio's eligibility is that his parents, Cuban immigrants, were permanent U.S. residents but had not yet been naturalized at the time of his birth. But Rubio didn't need his parents to be citizens to be born one himself. As Stephanopoulos's incredulous question to Trump suggested, the argument that Rubio should be disqualified is one that few take seriously.

Trump is always careful not to alienate any of his supporters, however — even those with fringe theories. As recently as Saturday, he offered new fodder for those who believe Obama is a closet Muslim.

The GOP front-runner usually stops short of saying flat out that he agrees with these ideas. But he's never one to dismiss a backer.