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.

Credibility problem

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.