WASHINGTON  -- Republican presidential front-runner Donald Trump said on Saturday it was not his job to stand up for U.S. President Barack Obama after a man at one of his campaign events said he 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.

Trump drew criticism on Friday from Republican and Democratic rivals in the 2016 race for the presidency when he failed to challenge a man at a New Hampshire town hall Thursday night who said Muslims were a problem in the United States.

"We know our current president is one. You know he's not even an American," the man said.

Trump, the billionaire television personality who leads the pack of Republicans seeking the presidential nomination, has cast doubt on whether Obama was born in the United States 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!"

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

Later on Saturday, Trump read out the tweets to audience applause at the Iowa Faith and Freedom Coalition Forum in Des Moines.

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 U.S. presidents. An unremarkable showing at Wednesday night's Republican debate, where the front-runner drew attacks from his many rivals, had some observers wondering if Trump frenzy is finally on the wane.

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

Parallels To 2008

In August, two Boston brothers charged with urinating and beating a homeless Mexican man told police they were inspired by Trump. "Donald Trump was right, all these illegals need to be deported," one said, according a police report.

On Monday, opponents and supporters clashed outside a rally for 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."

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

In Des Moines, Trump referred to the incident, saying: "McCain just ripped that microphone from that woman's hands ... I thought it was a little harsh."

David Axelrod, former White House adviser and Obama campaign strategist, said: "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."

"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, Trump appeared to reach for more in his tweets Saturday, implying that Christians have suffered under 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," Trump said.

For more on the 2016 presidential race, see the Reuters blog “Tales from the Trail.