Republican presidential frontrunner Donald Trump said on Saturday it was not his job to stand up for US 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 US presidents. An unremarkable showing at Wednesday night's Republican debate, where the frontrunner 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.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." the said, according a police report.