Media playback is unsupported on your device Media caption Michele Bachmann: "I have no regrets"

Minnesota congresswoman Michele Bachmann has ended her presidential campaign after finishing at the back of the field in Iowa's caucuses.

Her exit leaves six Republicans in the race to challenge Barack Obama for the White House in November.

Mitt Romney, who narrowly won in Iowa, meanwhile rolled out an endorsement from ex-Republican nominee John McCain.

Rick Perry, who came fifth, vowed to press on, but will focus on the next-but-one race in South Carolina.

Mitt Romney is a heavy favourite to win next week's primary election in the north-eastern state of New Hampshire.

Caucuses and primary elections will be held in all 50 states over the next six months before the Republican presidential nominee is confirmed at the party convention in Florida this August.

Former Massachusetts Governor Romney won Tuesday's Iowa caucuses by a wafer-thin margin over former Pennsylvania Senator Rick Santorum.

Analysis Conservative Republicans are still looking for a hero. If they were to come together around one anti-Mitt candidate, that person would win hands down. But no-one has been able to capitalise on initial success in the past, and there is no reason to think Mr Santorum is any different. There is a hunger on the right for something fresh and different. There's a mood in the country that the system is broken and doesn't work for ordinary people. Mr Romney is almost an incarnation of the establishment, from his well-manicured rallies to his tightly organised campaign lubricated with plenty of lucre, from his past as a successful businessman to his career as a problem-solving politician. He is not going to capture the public mood with that record, any more than he has won the heart of his party. Nail-biter opens Republican contest

He squeaked to victory in the early hours of Wednesday by just eight votes, with 30,015 ballots (24.55%) against 30,007 for social conservative Mr Santorum (24.54%).

Texas congressman Ron Paul, a libertarian-leaning anti-war candidate, came a strong third.

Mrs Bachmann, who came sixth with just 5% of the vote, told a Wednesday morning news conference in Des Moines, Iowa: "I have decided to stand aside."

The 55-year-old - who had courted the evangelical Christian vote and was briefly the front-runner back in August - said she had "no regrets".

Mrs Bachmann did not endorse another candidate, but said: "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 spoke at length about her reasons for entering the race, denouncing President Obama's healthcare reforms as "left-wing social engineering", which "endangered the very future" of the US.

A record 122,000 straw ballots were cast by Republicans in the largely rural state.

No-one will out-work him and we've yet to see whether coming second brings in a flood of money Prof Terry Madonna, Franklin and Marshall College, Pennsylvania Profile: Rick Santorum

There had been speculation that Texas Governor Rick Perry might also drop out after he said he would return to his home state to reassess his campaign.

But instead of going back to Texas, he tweeted on Wednesday: "Here we come South Carolina!!!"

The Perry campaign also confirmed he would attend a Republican debate on Saturday in New Hampshire.

The first primary of the 2012 election season will be held next Tuesday in the small New England state, which is Mr Romney's political heartland.

Campaigning there on Wednesday, Mr Romney rolled out another high-profile endorsement, this time from Arizona Senator John McCain.

Appearing alongside Mr Romney, Sen McCain told a crowd "no-one will ever say that Mitt Romney will lead from behind".

Sen McCain said the businessman's private sector experience would help turn around the economy.

"Our message to President Barack Obama is: 'You can run but you can't hide from your record.'"

Mr Romney is already spending hundreds of thousands of dollars on TV ads in New Hampshire and South Carolina.

And his well-financed political machine has snapped up television advertising space in the key swing state of Florida, which holds its primary at the end of this month.

Meanwhile, former House Speaker Newt Gingrich, who came fourth in Iowa, signalled that he would campaign more aggressively against Mr Romney, whom he has linked to a series of bruising TV attack ads.

The Gingrich campaign took out a full-page advert in the New Hampshire Union-Leader newspaper on Wednesday portraying their man as a "Bold Reagan Conservative" and Mr Romney as a "Timid Massachusetts moderate".

Appearing on US breakfast shows, Mr Romney acknowledged he was now a "big target" for attacks.

A Suffolk University opinion poll on Wednesday showed him with 43% of support in New Hampshire, while Mr Santorum was at just 6%.