WASHINGTON - Republican presidential candidate Ben Carson on Sunday said Muslims were unfit to be president of the United States, arguing their faith was inconsistent with American principles."I would not advocate that we put a Muslim in charge of this nation. I absolutely would not agree with that," Carson told NBC Meet the Press.Carson, a retired neurosurgeon who has been near the top of opinion polls for the crowded field of Republican candidates, said he thought a US president's faith should be "consistent with the Constitution."Asked if he thought Islam met this bar, Carson said: "No, I do not."Carson gave up some ground in a CNN/ORC poll released on Sunday, slipping to third place from second with 14 percent of support. Sixteen Republicans are seeking the party's nomination for the US presidential election in November 2016.The CNN/ORC poll showed real estate mogul Donald Trump continues to lead the Republican contest with the support of 24 percent of registered voters, down from 32 percent in a previous poll. Former Hewlett-Packard chief executive Carly Fiorina surged into second place with 15 percent support."It's obviously a very important moment because now more people know who I am," Fiorina told "Fox News Sunday." "We know, based on what's happened before this debate, that as people come to know me and they understand who I am and what I've done and most importantly what I will do they tend to support me."Trump, asked on CNN's "State of the Union" about the poll results, replied: "Well, I'm a little surprised, because other polls have come out where I actually picked up after the debate, I actually gained after the debate."On NBC's "Meet The Press" on Sunday, Trump was asked whether he'd accept a Muslim president, and replied: "Some people have said it already happened."