Presidential hopefuls George W Bush and John McCain clashed on television over negative advertising as Republicans debated live for the last time before Saturday's crucial South Carolina primary.

In a wide-ranging, 90-minute broadcast the two leading contenders and Republican outsider Alan Keyes also exposed differences on campaign funding reform, gay rights and foreign policy.

But the most heated exchanges came as both Mr Bush and Mr McCain produced examples of their opponent's advertising, which they each described as hurtful and insulting.

'Hurtful campaign'

Mr Bush accused the McCain campaign of striking below the belt with material which likened the Texas governor's character to that of President Bill Clinton.

"Don't compare me to Bill Clinton," said Mr Bush, adding that such comments were as low as a Republican could go.

Mr McCain hit back saying a speaker at a Bush campaign event had launched a personal attack, saying the former war hero had neglected war veterans.

"I don't know if you can understand, this, George, but that really hurt. That really hurt," the Arizona Senator said.

As voices rose the third contender, Alan Keyes interjected: "Is this the kind of spectacle that actually characterises a serious political process, because I don't think it does."

Foreign policy changes

Mr Bush singled out relations with China as an area of US foreign policy in need of reform.

"The current president has called the relationship with China a strategic partnership," he said.

"I believe our relationship needs to be redefined as one as competitor.

"We must make it clear...that we don't appreciate any attempt to spread weapons of mass destruction around the world, that we don't appreciate any threats to our friends and allies in the Far East."

Senator McCain said he would institute a policy which he called "rogue state rollback".

Pointing to Iraq, Libya and North Korea, he called for resources to be aimed at the eventual overthrow of their governments and the installation of "free and democratically elected" alternatives.

Campaign reform

George W Bush highlighted his policies on campaign reform, a central plank of the McCain campaign, denying he had only lately come to have a view on the issue and promising to stop so-called "soft money" contributions from big business and unions.

But Mr McCain said there was a "$1bn loophole" in the Texas governor's plans, which he said failed to combat million-dollar personal contributions.

On abortion and gay rights, both of the leading contenders sought to distance themselves from hypothetical questions.

One-time Republican vice presidential candidate Dan Quayle once conceded he would accompany his daughter to an abortion clinic in certain circumstances although he would in office be a staunch pro-lifer.

But Mr Keyes said he opposed the presence of gay men and women in the military and called for an end to the current "Don't ask, don't tell" policy introduced by the Clinton administration.