Michele Bachmann, the Minnesota congresswoman, has ended her bid to win the Republican US presidential nomination after coming in last in the the Iowa Republican caucuses.
She has asked Republicans to unite behind the party's presidential nominee to roll back President Obama's healthcare overhaul and to "take back our country".
"I have decided to stand aside ... I will not be continuing in this race for the presidency," Bachmann, who secured only five per cent of the vote, announced in Des Moines, Iowa, on Wednesday.
She said "President Obama and his socialist policies must be stopped" and that she was certain the country's founders were "watching" her.
Bachmann, a favourite of the conservative Tea Party group, declined to disclose whom she would endorse. "I believe that we must rally around the person that our country and our party and our people select to be that standard bearer," she said.
Mitt Romney, the former Massachusetts governor, won the Iowa caucuses by a narrow margin of votes over the former Pennsylvania senator, Rick Santorum.
He also won the endorsement of John McCain, the Arizona senator who lost the White House race to Obama in 2008.
"I'm really here for one reason and one reason only, and that is to make sure that we make Mitt Romney the next president of the United States of America," McCain told cheering supporters in a high school gymnasium in New Hampshire on Wednesday.
Romney edged out Santorum by only eight votes, winning by 30,015 to 30,007, Republican Party officials in the midwestern state announced.
Wednesday's result will solidify Romney's front-runner status, as the Republicans head for their next contest in his stronghold of New Hampshire.
Iowa caucuses were the first contest in the state-by-state battle for the Republican presidential nomination to face Obama on November 6.
Both candidates were locked in a neck-and-neck fight till the end at 25 per cent votes. Ron Paul, a Texas congressman, claimed third place with 21 per cent of the vote.
Sixty-four-year-old Romney is a former governor of Massachusetts. He is a strong supporter of the military and advocates boosting troop numbers.
Romney said he and Santorum each had "a great victory" and congratulated Paul, then attacked Obama, saying: "This has been a failed presidency."
Fifty three-year-old Santorum has gone from outsider to one of the leading candidates in Iowa.
Elected to the senate in 1994, Santorum is a fierce opponent of abortion and gay rights. He would push to ban same-sex marriage and penalise employers who hire illegal immigrants.
Speaking to Al Jazeera, John Nichols, a political writer for The Nation magazine, said: "I don’t think one would write Santorum off as uncompetitive. He could be a serious contender.
"But to get there, he is going to have to work his way through a really intense Republican primary process.
"And that will be test if he can beat Romney, which is an uphill race. I think he can come into contest with Obama as something like a giant killer may be junior Ronald Reagan. But he has still to beat Romney."
Paul's unorthodox views, including supporting a return to the gold standard and an end to a US overseas military presence, have won him a passionate following among voters who feel deeply alienated from more mainstream candidates.
"This movement is going to continue and we're going to keep scoring just as we have tonight," he said.
Newt Gingrich, the former US House of Representatives speaker, got 13 per cent votes,
Rick Perry, the Texas governor, collected 10 per cent of votes. The seventh candidate, Jon Huntsman, a former Utah governor, did not campaign in Iowa.
Perry, who was relegated to fifth position, said he "will return to his home state of Texas to assess presidential bid".