Donald Trump's old campaign against US President Barack Obama has come back to bite him.

Democratic candidate Hillary Clinton landed one of the biggest blows in the first US presidential debate when she mocked Mr Trump's claim that Mr Obama wasn't an American at all and the suggestion that he was actually born in Kenya.

Clinton leapt on the controversy to attack Mr Trump's character.

"He has really started his political activity based on this racist lie that our first black president was not an American citizen," Mrs Clinton said.


"There was absolutely no evidence for it but he persisted, he persisted year after year because some of his supporters, people that he was trying to bring into his fold, apparently believed it or wanted to believe it.

"I like to remember what Michelle Obama said in her amazing speech at our Democratic National Convention. 'When they go low, we go high.' And Barack Obama went high despite Donald Trump's best efforts to bring him down."

For several years, Mr Trump was the leader of the so-called birther movement, whose members believe Mr Obama wasn't born in the United States.

Mr Obama produced his birth certificate to put the issue to bed in 2011, but Mr Trump didn't admit he was born in the US until a few weeks ago.

That all came back to bite him, as moderator Lester Holt brought it up in the debate.

"Mr Trump, for five years you perpetuated a false claim of the nation's first black president wasn't a natural born citizen. You questioned his legitimacy," Holt said.

Mr Trump replied by claiming a Clinton ally, Sid Blumenthal, had actually raised the birther conspiracy first.

"Blumenthal sent a highly respected reporter to Kenya to find out about it. They were pressing it very hard. She failed to get the birth certificate. When I got involved. I didn't fail. I got him to give the birth certificate," Mr Trump said.

"The birth certificate was produced in 2011. You continued to question the legitimacy as recently as January. So what changed your mind?" Holt asked.

"Nobody was pressing it. Nobody was caring much about it. I figured you'd ask the question tonight, of course. But nobody was caring much about it. But I was the one that got him to produce the birth certificate and I think I did a good job," Mr Trump said.