By the BBC's Gordon Corera

It was hard to tell whether it was the realisation of what he was about to say that caused John McCain to stumble over one of the sentences in his South Carolina concession speech or whether it was simply sheer exhaustion.

Eventually he got the line out: "I will never dishonour the nation I love or myself by letting ambition overcome principle. Never, never, never".

It was not the only barb in a speech that surprised many observers with its uncompromising attacks on George W. Bush.

"I will not take the low road to the highest office in this land. I want the presidency in the best way not the worst way" said the defeated Arizona Senator.

Even more starkly he portrayed the battle between himself and Bush as "a choice between experience and pretence."

Tactical blunders

The scale of his defeat - 11 points - undoubtedly shook McCain but the speech served only to display some of the pitfalls he has faced in this primary campaign and the reasons why he may not become the Republican nominee.

McCain lost South Carolina partly because of tactical blunders.

He came out of New Hampshire with the image of not being a 'typical politician' - instead he was a straight-talker who told the truth and could broaden out the Republican Party by bringing in new voters.

But instead of being able to fight the kind of independent-minded campaign he managed in New Hampshire, McCain fell into Bush's trap when, in response to attacks by Bush, he launched TV ads attacking Bush, including one that compared the Texas Governor to Clinton in terms of their truthfulness.

Even worse, Bush managed to portray McCain as the one who started it.

Worst of both worlds

By the time McCain realised that he was losing his "outsider image" it was too late and the damage was done.

By pulling back, he ended up with the worst of both worlds - having already lost his reputation as being "above" the normal type of politics, he was then left undefended against one of the most sustained, massive, and nasty negative campaigns in recent primary history.

Bush's tactics were so effective that even those constituencies expected to back McCain most strongly did not deliver - according to exit polls even veterans broke evenly between the two.

More voters also thought Bush was a reformer than McCain and higher percentages thought McCain was "unfair" in his attacks than thought Bush was at fault.

In a sign of the power of negative attacks, 50% of voters thought that McCain lacked the ethical standards to serve effectively as President, while only 21% thought the same of Bush.

Core conservatives

Crucially, Bush managed to mobilise core conservative Republican voters, warning that McCain planned to swamp them with Democrats and Independents on election day.

With his emphasis on conservative values, Bush won 68% of the vote from members of the religious right who made up a third of the primary electorate.

But it's not just John McCain who has been damaged by the experience.

At his victory speech, Bush began by saying that the result was "the victory of a message that is compassionate and conservative".

The line reflected the knowledge in the Bush camp that South Carolina has pushed them far further to the right than they had expected.

Lost message

The original message - that Bush was a centrist, "compassionate conservative" - had been lost in the drive to mobilise the Christian and conservative base of support in South Carolina.

In polls, Al Gore has been catching up with Bush rapidly in a potential general election race and South Carolina will have given the Democrats far more ammunition to use against Bush.

"We have united our cause and our party" said Bush, but the problem is that while he has got the clear backing of his party, it is not clear he will now have the message and credibility to win over the rest of the electorate.

That is especially the case with the centrist voters - the prime target in general election campaigns.

These are exactly the type of voters who have been backing John McCain so far and Bush will now have his work cut out to win them back when it comes to the general election race.

South Carolina's primary was a bruising experience for both Republican candidates and both will surely be happy to move on.