New Jersey Gov. Chris Christie says GOP front-runner Donald Trump must decide "how serious a candidate he wants to be," after failing to correct a questioner who said President Obama was a Muslim and not a U.S. citizen. "I wouldn't have permitted that," Christie responded when asked how he would have handled the same situation. "I would correct him. I'd say that the president's a Christian and he was born in this country. Those two things are self-evident." Thursday night in New Hampshire, in front of about 3,000 fans, the first questioner told Trump, "We have a problem in this...