Trump says he's 'not sure' if his opponents can legally be president

Republican presidential candidate, and front-runner, Donald Trump is "not sure" if his main opponents are even eligible to run for president.

On Saturday, Trump retweeted a message in which a follower said both Marco Rubio and Ted Cruz were ineligible.

SEE MORE: COMPLETE 2016 ELECTION COVERAGE FROM AOL.com

"It was a retweet -- not so much with Marco. I'm not really that familiar with Marco's circumstance," Trump said on ABC.

Click through some of Donald Trump's most controversial Tweets:

"But then why retweet it?" said George Stephanopoulos. "Because I'm not sure. I mean, let people make their own determination," Trump said.

Rubio was born in Miami to immigrant parents and Cruz in Canada to an American mother. Candidates don't actually have to prove eligibility unless they're challenged through a lawsuit.

Both Republican candidates have faced several of these cases, but all, so far, have been dismissed.

The legal case revolves around the requirement the president be a "natural born citizen," a phrase which isn't defined in the Constitution.

Both candidates have argued against a strict reading of the phrase, with Rubio saying it "would jeopardize centuries of precedent and deem at least six former presidents ineligible for office."

"The law under the Constitution, and federal law, has been clear from the very first days of the Republic. The child of a U.S. citizen born abroad is a natural born citizen," Cruz said at a town hall meeting.

Trump's eligibility questions go beyond the Republican race. In January, he told CNN someday he'll write a book on his theory regarding President Obama's eligibility.

More from AOL.com:
Two incidents with racial undertones stirring anger on UW-Whitewater campus
Man gets locked up for at least 6 months over $1 robbery
Mississippi police officer killed, three wounded after standoff