The country's most prominent Republican birther says that one of his party's early frontrunners for the 2016 presidential nomination may "perhaps not" be eligible to run if he was born in Canada.

In an interview that aired on Sunday, ABC's Jonathan Karl reminded billionaire Donald Trump that his false claim that President Barack Obama was not born in the United States made him appear "not serious."

"You don't still question whether he was born the United States?" Karl wondered.

"I have no idea," Trump said. "Well, I don't know. Was there a birth certificate? You tell me. You know, some people say that was not his birth certificate. I'm saying, I don't know, nobody knows. And you don't know either, Jonathan. You're a smart guy. You don't either."

"I'm pretty convinced he was born in the United States," Karl replied.

"Pretty! Ah! Pretty! Pretty! No! No! You said, pretty!" Trump exclaimed.

"Totally without question that he was born in the United States," Karl said, correcting his earlier language.

"You said, you're pretty convinced," Trump insisted. "Okay? So, let's just see what happens over time."

Karl pointed out that Sen. Ted Cruz (R-TX) had been born in Canada and asked if that made him ineligible to be president of the United States.

"If he was born in Canada, perhaps not," Trump admitted. "But I'm not sure where he was born."

"Oh, he was definitely born in Canada," Karl noted.

"Okay, well, then you'll have to ask him that question, but perhaps not," Trump said. "Look, that will be ironed out. I don't know the circumstances. I heard somebody told me he was born in Canada. That's really his thing."

Watch this video from ABC's This Week, uploaded Aug. 11, 2013.