Republican presidential frontrunner Donald Trump has said it was not his job to stand up for Barack Obama after a man at one of his campaign events said the president was a Muslim and “not even an American”.

“Am I morally obligated to defend the president every time somebody says something bad or controversial about him? I don’t think so!” Trump said on Twitter.

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Mr Trump drew criticism from Republican and Democratic rivals in the White House race when he failed to challenge a man at a New Hampshire town hall who said Muslims were a problem in the US.“We know our current president is one. You know he’s not even an American,” the man said.

When Trump announced his presidential run many dismissed him, but he’s popular with voters.

Trump, the billionaire television personality who leads the pack of Republicans seeking the presidential nomination, has cast doubt on whether Mr Obama was born in the US and therefore qualified to stand as president.

“This is the first time in my life that I have caused controversy by NOT saying something,” Trump tweeted.

“If someone made a nasty or controversial statement about me to the president, do you really think he would come to my rescue? No chance!”

Mr Obama is a Christian who as president has attended church occasionally.

Mr Trump rattled the Republican establishment with a summer surge to the top of the polls, overshadowing expected favorite Jeb Bush, the son and brother of two presidents.

His anti-immigrant rhetoric, described as divisive even by members of his own party, has tapped into a vein of anger among like-minded supporters.

‘Obama has been horrible, I will be great’

In August, two Boston brothers charged with urinating and beating a homeless Mexican man told police they were inspired by Mr Trump.

“Donald Trump was right, all these illegals need to be deported.” they said, according a police report.

Last week, opponents and supporters clashed outside a rally for Mr Trump. Protesters, many of them Hispanic, shouted “Shame on you”, while some Trump fans retorted, “Keep them out”.

The tenor of the campaign has drawn parallels to the 2008 rallies by Republican presidential candidate John McCain and his running mate, Sarah Palin, that drew virulent anti-Obama rhetoric including shouts of “terrorist” or “traitor”.

Senator McCain eventually had to confront the issue at an October campaign rally where a woman called Mr Obama an Arab. He stopped her and called Mr Obama a decent family man.

“I think a lot of the people who are supporting Donald Trump are the same people who showed up at those Sarah Palin rallies at the end of the 2008 campaign,” said David Axelrod, former White House adviser and Obama campaign strategist.

“We know that there is this cohort out there who believe that we are being encroached on by illegal aliens, Muslims, China and Trump has tapped into that constituency,” he said on CNN.

Far from trying to tamp down the controversy, Mr Trump appeared to reach for more in his latest tweets morning, implying that Christians have suffered under Mr Obama.

“Christians need support in our country [and around the world], their religious liberty is at stake! Obama has been horrible, I will be great,” Mr Trump said.