Trump's retweet. ABC/screenshot Donald Trump said Sunday that he was "not sure" whether his Republican presidential rival Sen. Marco Rubio of Florida was eligible to be president.

"I don't know. I've never looked at it, George. Honestly, I've never looked at it," Trump told "This Week" host George Stephanopoulos.

Stephanopoulos had asked Trump about a tweet he shared the day before. The tweet said both Rubio and Sen. Ted Cruz of Texas were legally barred from becoming president.

"It's a SLAM DUNK CASE!! Check it!" the tweet said.

Trump's tweet included a link to a video in which a woman argued that because the Constitution requires that presidents be "natural born" citizens, only people whose parents were US citizens were eligible.

Rubio was born in the US to Cuban parents. Cruz was born in Canada to an American mother and a Cuban father.

Trump, whose mother was from Scotland, insisted to Stephanopoulos that because his post was only a retweet, he wasn't personally endorsing the message. Trump did argue, however, that Cruz had a "problem" with his eligibility.

"I think the lawyers have to determine it. It was a retweet. Not so much with Marco. I'm really not familiar with Marco's circumstance," he said, pivoting his answer to Cruz.

"I'm not sure. Let people make their own determination," he added.

Donald Trump. REUTERS/Tami Chappell

Stephanopoulos continued to press Trump on the topic. Though a handful of prominent legal scholars have questioned Cruz's eligibility, only fringe activists have questioned Rubio's.

"You're really not sure that Marco Rubio is eligible to run for president? You're really not sure?" Stephanopoulos asked Trump.

"Somebody said he's not and I retweeted it," Trump said. "I have 14 million people — between Twitter and Facebook and Instagram — and I retweet things and we start dialogue. And it's very interesting."

For his part, Rubio dismissed Trump's potential concern over his eligibility for the Oval Office.

"This is a game he plays: He says something that's edgy and then the media flocks and covers that … It's not going to work anymore," Rubio said in another interview on ABC.

Watch Trump on 'This Week' below: