John Bolton, a U.S. ambassador to the United Nations during the Bush administration, endorsed Mitt Romney on Thursday, adding another name to the list of conservative politicians who have come out in support of the former Massachusetts governor in an attempt to reassure voters who see Romney as a flip-flopping moderate.

It was also something of a snub to Newt Gingrich, who had indicated that, if elected president, he would make Bolton his secretary of state.

Of all the candidates, Mitt Romney possesses the strongest vision for America's leadership role in the world, and I am proud to endorse him, Bolton said in a statement.

Bolton, who worked in the State Department before holding his U.N. post in 2005 and 2006 under President George W. Bush, is known for his hawkish foreign-policy views and for his involvement with a wide range of conservative groups, including the American Enterprise Institute, the Project for the New American Century and the National Rifle Association. He supports the Tea Party and is strongly identified with the neoconservative movement, although he has personally rejected that label.

Despite such credentials, Bolton's endorsement is unlikely to make much of a difference in Romney's support. This is partly because his name recognition is limited, but more so because he is only the latest in a string of conservative endorsements, all of which have combined to bolster Romney's credibility somewhat but none of which have been particularly influential on their own.

Romney's endorsements list includes Sen. Kelly Ayotte of New Hampshire, Sen. John Thune of South Dakota, Gov. Nikki Haley of South Carolina and 2010 Senate candidate Christine O'Donnell of Delaware, all of whom are known for their conservative views and for their affiliation with the Tea Party, whose supporters are among the least likely to back Romney. Bolton's endorsement adds another conservative name to this list, but it does not break new ground.