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!"