Democratic presidential hopeful John Kerry has won the backing of his former rival Wesley Clark in his campaign to take on President Bush in November.
Former rival Wesley Clark (right) has backed Mr Kerry
Mr Clark told a rally in Wisconsin that he would do all he could to help get George W Bush out of the White House.
Mr Kerry is hoping to cement his lead in the Democratic race with a victory in Nevada on Saturday.
Mr Clark dropped out of the race earlier this week after winning just one state of the 14 contested so far.
Bush campaign video
With opinion polls currently showing Mr Kerry running ahead of Mr Bush, the Bush re-election campaign on Thursday released a video accusing the Massachusetts senator of being "unprincipled".
The video was posted on Mr Bush's campaign website and a link sent to six million email addresses.
The video counters Mr Kerry's oft-repeated claim that he would show special interests the door, by saying that "the only door he's shown special interests is the front door of his office".
Mr Bush's paid television advertising is not expected to begin until late March or April.
On Friday Senator Kerry dismissed a claim by an internet gossip site that he had had an affair with a female intern.
"Well there is nothing to report, so there is nothing to talk about," he told MSNBC television. "There's nothing there. There's no story."
And he said he was ready for attacks from Republicans.
"These guys will want to try to do everything to change the subject," he said in an interview on the Don Imus radio programme.
"But the subject is three million jobs lost in the last few years, it's health care that's completely unaffordable and unreachable for tens of millions of Americans, it's kids
who are not getting a decent education all across America."
Mr Kerry has so won far won 12 state contests to pick the Democrat candidate for the November election.
He is now hoping for victories in Nevada, which is holding caucuses on Saturday, and in Wisconsin, which holds its primary
Mr Clark's backing will be seen as a new boost for Mr Kerry.
"John, I'll work with you to do everything I can to help you take the White House back for its rightful owners, the American people," Mr Clark told Mr Kerry at a rally in Madison, Wisconsin.
Rivals still fighting for the Democratic nomination are Senator John Edwards, former Vermont governor Howard Dean, civil rights activist Al Sharpton and Ohio congressman Dennis Kucinich.