Property mogul Donald Trump has suggested that US president Barack Obama had been a poor student who did not deserve to be admitted to the Ivy League universities he attended.
Mr Trump, who is considering a bid for the Republican presidential nomination, offered no proof for his claim but said he would continue to press the matter as he had the legitimacy of the president's birth certificate.
"I heard he was a terrible student, terrible. How does a bad student go to Columbia and then to Harvard?" Trump said in an interview with The Associated Press.
"I'm thinking about it, I'm certainly looking into it. Let him show his records."
Mr Obama graduated from Columbia University in New York in 1983 with a degree in political science after transferring from Occidental College in California. He went on to Harvard Law School, where he graduated magna cum laude - with great honour - in 1991 and was the first black president of the Harvard Law Review.
His 2008 campaign did not release his college transcripts and in his best-selling autobiography Dreams From My Father, Mr Obama indicated he had not always been an academic star.
Mr Trump said Mr Obama's refusal to release his college grades were part of a pattern of concealing information about himself.
"I have friends who have smart sons with great marks, great boards, great everything and they can't get into Harvard," he said. "We don't know a thing about this guy. There are a lot of questions that are unanswered about our president."
Katie Hogan, a spokeswoman for Mr Obama's re-election campaign, declined to comment.
Mr Trump has shaped himself as an ultra-conservative candidate, reversing some positions he once held. He now would make abortion illegal and opposes gay marriage and gun control.
He advocates repeal of Mr Obama's health care overhaul that became law last year, wants to cut foreign aid, is highly critical of China's trade and monetary policies and wants to end the US wars in Iraq and Afghanistan.
But he has got the most political traction by latching on to the "birther" movement: those who believe claims initiated by the far-right that Mr Obama was born outside the US - despite the release of official birth records in Hawaii and other evidence. The US Constitution requires that presidential candidates be "natural-born" US citizens.
Of late, Mr Trump has appeared in interviews on all the major American cable television networks, pushing relentlessly his message that Mr Obama needs to prove he was born in the US.
"I have more people that are excited about the fact that I reinvigorated this whole issue," he said, adding "the last guy (Mr Obama) wants to run against is Donald Trump".
Mr Trump will travel to the early primary states of New Hampshire and Nevada this week and says he will make a final decision about a presidential bid by June.
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