KEY POINTS:
Senators Hillary Clinton and Barack Obama are getting down and dirty as the battle to become the Democrat candidate in next year's presidential election heats up.
An ugly fight between the two front-runners raged all last week. Previous spats had broken out between campaign officials, but now the candidates are openly attacking each other.
There are signs that the fight has damaged both the top-tier candidates and perhaps opened the door to another Democrat contender.
A CNN poll in the first voting state of Iowa showed Clinton and Obama had dropped 6 per cent each to second and third place respectively.
Left-wing candidate John Edwards led the Iowa poll by five points, though he trails Obama and Clinton in all national polls.
The duration and viciousness of the Clinton-Obama clash has caught Washington by surprise, and throws serious doubt on whether Clinton could choose Obama as a running mate if - as many expect - she wins the nomination.
The fight began at the start of the week during the YouTube debate in South Carolina, in which all the Democratic candidates were asked questions submitted through the video-sharing website.
Obama answered a question on foreign policy by saying he would meet leaders of nations considered unfriendly to America. Clinton said she would not.
Later, Clinton described Obama's stance as "naive".
That remark appeared to land a direct hit on Obama's campaign. Clinton aides and the Republican Party have repeatedly identified Obama's relative inexperience as his weakness.
But Obama immediately went on the offensive.
His campaign's central message is one of change and he slammed Clinton's foreign policy ideas as a mere continuation of the current administration's.
He dubbed her "Bush-Cheney Lite", and attacked her most vulnerable spot, her early support for the invasion of Iraq.
That brought further attacks from Clinton.
It is hard to pick a winner in the dispute. Clinton has shown she can pick holes in the campaign of her most dangerous rival.
Obama has shown he will immediately go on the offensive when attacked.
But the real winners could be Edwards and the other second-tier Democratic candidates.
The fight gives those candidates a chance to boost their own profiles and pick up disaffected supporters from both campaigns.
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