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Boost for birthers as poll reveals only 38% of Americans believe Obama was born in U.S. (but it isn't much higher for Trump).
His apparent obsession with seeing the president's birth certificate has attracted the wrath of politicians and actors - not least Robert De Niro, who likened him to a 'car salesman'.
But Donald Trump's relentless birther campaign received a significant boost today when a poll revealed fewer than four in ten American voters believe Barack Obama was definitely born in the U.S.
But the poll showed Mr Trump's own background is also under the spotlight - fewer than half those who completed it said they were sure he was born on American soil.
The results emerged as the entrepreneur - and potential 2012 presidential candidate - hit back at Godfather star De Niro, calling him 'not the brightest bulb on the planet.'
The 64-year-old property tycoon also suggested in an interview today that President Barack Obama was a poor student who did not deserve to be admitted into the Ivy League universities he attended.
His high-profile allegations about the president's origins appear to have gained traction with voters, with 38 per cent saying they are not convinced he was born in America, according to the USA Today-commissioned poll.
But the poll also revealed a blow to Mr Trump's White House ambitions.
Despite the fact the Celebrity Apprentice star scores top of Republican polls for 2012 presidential nominations, 50 per cent of Americans, including 31 per cent of Republicans, said he would make a 'poor' or 'terrible' president.
It seems the birther issue isn't driving support for Mr Trump, either. Among those who say they definitely or might vote for Mr Trump just a third question Mr Obama's birthplace.
According to the poll, voters who believe the president wasn't born in Hawaii tend to support Mr Trump's Republican rivals Sarah Palin, Mitt Romney and Mike Huckabee.
He became embroiled in an unlikely slanging match with De Niro this week after the Hollywood star added his voice to a growing band of celebrities and political figures who are slamming Mr Trump for his birther 'obsession'.
When Mr Trump was asked to comment on the criticism, he went on the offensive, telling Fox News 'well, he's not the brightest bulb in the planet.'
He added: 'I have been watching over the years and I like his acting, but in terms of when I watch him doing interviews and various other things, we are not dealing with Albert Einstein.
'He can say what he wants, but the fact is [Mr Obama] has not revealed his birth certificate.'
De Niro was interviewed by NBC News anchor Brian Williams at the Tribeca Film Festival on Saturday.
The actor said: 'I won't mention names, but there are certain people on the news in the last couple of weeks - what they're doing is crazy.
'They're making statements about people that they don't even back up. Go get the facts before you start saying things about people.'
When Mr Williams asked whether 'any of those people have shows on my network on Sunday night?' - a reference to Mr Trump's Celebrity Apprentice - De Niro replied: 'Yes. It's like a big hustle. It's like being a car salesman.
'Don't go out there and say things unless you can back them up. How dare you? It's awful. Just go out there and speak and say these terrible things? It's crazy.'
Mr Trump has also been criticised by New York Mayor Michael Bloomberg and comedian Jerry Seinfeld, who pulled out of an event with Mr Trump over the issue.
Today the entrepreneur offered no proof for his claim about Mr Obama's grades, but said he would continue to press the matter as he had the legitimacy of the president's birth certificate.
Mr Trump said: 'I heard he was a terrible student, terrible. How does a bad student go to Columbia and then to Harvard? 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 1991 and was the first black president of the Harvard Law Review.
Mr Obama's 2008 campaign did not release his college transcripts, and in his best-selling memoir, Dreams From My Father, Mr Obama indicated he hadn't always been an academic star.
Mr Trump said Mr Obama's refusal to release his college grades was part of a pattern of concealing information about himself.
He added: 'I have friends who have smart sons with great marks, great boards, great everything and they can't get into Harvard.
'We don't know a thing about this guy. There are a lot of questions that are unanswered about our president.'
The truth at last? Investigation claims president WAS born in Hawaii
The birther campaign was seriously undermined last night by an in-depth CNN investigation, which appeared to offer final proof the president was born in Hawaii.
Interviewers travelled to Hawaii, where they spoke to Dr Chiyome Fukino, a former director of the state's Department of Health - and staunch Republican.
In her first TV interview, she said she has seen the long-form version of his birth certificate in the department's vaults and has 'no doubt' Mr Obama was born in the state.
She said the certificate is 'absolutely authentic... he was absolutely born here in the state of Hawaii.'
It counters an allegation made by Mr Trump in an interview with the channel last night, when he claimed the certificate was missing - but wouldn't say how he knew.
Supporters of the conspiracy claim only this is legal proof of his birth, dismissing the computer generated 'certification of live birth' he released during the 2008 election campaign.
But officials told CNN the fuller version is in fact no longer legally accepted, and can only be obtained by a lengthy Freedom of Information request.
When any Hawaiian citizen asks for a copy of their birth certificate they receive the same short-form computer generated version as Mr Obama, certified for all legal purposes.
The channel even took a Hawaiian man born the day after Mr Obama to pick up a copy of his birth certificate. Like Mr Obama, Stig Waidelich was given a computer generated version.
His birth was advertised in the same August 1961 edition of the Honolulu Star-Bulletin as Mr Obama's, an announcement many birthers have condemned as fake.
But CNN spoke to a long-time reporter on the newspaper, Dan Nakaso, who said it wasn't possible because information came directly from the Department of Health.
Interviewers also spoke to people on the island who remember Mr Obama being born, including state governor Neil Abercrombie, who once again recalled celebrating his birth.
Mr Waidelich's mother, Monika, said she believes she saw the future president in Honolulu's Kapi'olani Medical Center next to her son. She said: 'In those days, there were hardly any other black babies.'
Professor Alice Dewey, a faculty advisor to his mother, Ann Dunham, at the University of Hawaii, recalled a conversation in which Mrs Dunham compared the birth of Mr Obama to that of his sister, Maya.
The professor said: 'She said, "When I had Maya, it was a lot of more difficult because Indonesia doesn't believe in painkillers while you're giving birth. ... Of course, in the United States, giving birth to Barry (Obama's childhood nickname) was quite different and much more comfortable.".'