Senator Hillary Clinton returned to political life today as her husband, former president Bill Clinton, gave a terse endorsement of Democratic White House hopeful Barack Obama.

The New York senator made her first appearance in Congress since losing the Democratic nominating race to Obama three weeks ago.

Clinton appeared on Capitol Hill for a luncheon with her Democratic Senate colleagues, ahead of her first joint campaign rally with Obama in the aptly named New Hampshire town of Unity on Friday.

The former first lady was greeted by interns and staffers crowded outside the Congress building in brilliant sunshine, and did not comment to reporters before heading in for the luncheon.

Her husband, the former president, meanwhile issued a one-sentence statement through his spokesman Matt McKenna to belatedly back Obama after months of fireworks on the campaign trail.

"President Clinton is obviously committed to doing whatever he can and is asked to do to ensure Senator Obama is the next president of the United States," McKenna said.

Bill Clinton is not scheduled to speak at Friday's event. He was tight-lipped on June 7, when his wife gave a rousing concession speech and promised to work all-out for Obama.

Obama spokesman Bill Burton said the Illinois senator and Hillary Clinton had spoken Sunday to discuss how the new Democratic standard-bearer could liaise with the former president in future.

"A unified Democratic Party is going to be a powerful force for change this year and we're confident president Clinton will play a big role in that," Burton said.

Hillary Clinton drummed home the message of unity Monday as she posted a video on her website, appealing for donors to help pay down her campaign debts of $US22.5 million ($23.66 million) - half of which she lent to the campaign herself.

"Together we made history and I will continue to work toward our common goal of building an America that respects and embraces the potential of every last one of us," Clinton said in the video.

"This goal is shared by our Democratic Party nominee, Senator Barack Obama, and I look forward to campaigning with him across this great country of ours," she said.

Today, Clinton was due to attend a meeting of Democratic members of the House of Representatives.

On Thursday, after addressing a conference of Latino politicians, she is scheduled to introduce Obama to a private meeting of her leading fundraisers at a Washington hotel.

AFP