By Steve Schifferes
After his poor showing on Super Tuesday, former Massachusetts governor Mitt Romney has effectively left the race, clearing John McCain's path to the Republican presidential nomination.
So what went wrong with Mr Romney's well-financed campaign?
Mr Romney had attempted to run as the legitimate heir of Ronald Reagan, one of the most popular Republican presidents in recent times, a "conservative's conservative."
He had the looks, the personality and the money to be a credible Republican presidential candidate.
And as a Republican governor in a Democratic state, he appeared to have the potential to reach out beyond the Republican base.
But Mr Romney had a big credibility problem among Republican primary voters, who were not convinced by his conversion from a liberal Massachusetts governor to a conservative candidate for national office.
His "flip-flop" on a universal health care mandate, which he had introduced in Massachusetts but repudiated nationally, was one commonly cited example.
Conservatives were also suspicious about his changed views on social issues like abortion.
This served to alienate him from the Christian right, a key group within the Republican party, which was already suspicious of his Mormon religion.
In the primary elections on Tuesday, most Christian evangelicals backed Mike Huckabee, taking enough votes away from Mr Romney to give victory to John McCain in key states like California and New York.
Although he won a number of smaller states, especially those who chose their delegates by caucus, his weak showing in his home state of Massachusetts, which he only narrowly carried, was an early indication of his fading campaign.
Mr Romney's best showing came in Michigan, where his father had been a popular governor, and where he backed extra help for the hard-pressed car industry.
But ironically, as the economy became the dominant issue in the election, even among Republicans, Mr Romney did not gain the expected boost from his business background.
Instead, he was attacked by Mr McCain and Mr Huckabee as a business executive who had little sympathy for the average worker and his troubles.
Mr Romney's role as head of the consultancy Bain left him with plenty of money to finance his own presidential campaign, and he never faced the fundraising difficulties of Mr McCain and Mr Huckabee.
He was also able to mobilise the support of many right-wing Republican commentators who have played a key role in mobilising grassroots activists in the party.
But the failure of his well-financed campaign to galvanize that base means that the Republican right now lacks a credible standard bearer to fight its cause.
Mr Romney's lukewarm endorsement of Mr McCain, whom he praised only for his stand on Iraq, shows that the Republican civil war has not ended with his retirement from the field.
Mr Romney said he was standing down in the best interests of his party.
But in the end, his comfortable circumstances may have meant he did not have the fire in his belly to continue what looked like an increasingly hopeless campaign.