Hillary Clinton today announced she will formally congratulate Barack Obama as the victorious Democratic presidential nominee at a Washington rally on Saturday, as the Obama camp resists her demand to be his running mate.
In a letter to supporters today, Clinton said: "On Saturday, I will extend my congratulations to Senator Obama and my support for his candidacy.
"I have said throughout the campaign that I would strongly support Senator Obama if he were the Democratic party's nominee, and I intend to deliver on that promise."
Clinton confirmed she would hold the event in Washington to thank everyone who backed her campaign. It was originally planned for Friday, but has been switched so more supporters can attend.
The Obama camp, though anxious to pull the party together after a bitter contest, is hostile to the idea of having Clinton on a joint ticket, fearing that having her and Bill Clinton at the heart of the presidential campaign and in the White House would be disruptive.
Clinton's letter to supporters came after signs of rising frustration among Democratic members of Congress at her refusal to concede the race or congratulate Obama. They feared it could hurt the party's prospects against the Republican candidate, John McCain, in November.
Clinton aides said she made the decision after a late afternoon conference call with 22 Democratic leaders who had supported her campaign. One of those supporters, the New York member of Congress Charlie Rangel, had made his anger public, chiding Clinton in TV interviews for failing to acknowledge Obama's victory.
Clinton told staff at her campaign headquarters to stop coming to work from tomorrow.
The issue of how to handle Clinton has become Obama's first big test as the Democratic nominee.
In an indication that he was preparing to snub her for the vice-presidential nomination, Obama confirmed he is commissioning three prominent Democrats to vet potential candidates. They are Caroline Kennedy, a member of one of America's most prominent political families and an early Obama supporter; Jim Johnson, a Democratic businessman; and Eric Holder, a deputy attorney general in the Clinton administration.
Obama's spokesman, Bill Burton, said: "Senator Obama is pleased to have three talented and dedicated individuals managing this rigorous process."
Obama became the presumptive Democratic nominee on Tuesday night after the last two contests of a race that spanned 54 primaries and caucuses beginning in January. He won Montana 56% to 42%, but Clinton took South Dakota 56% to 44%. The share of delegates he won in these contests pushed him over the finishing line of 2,118 delegates. He is to be formally adopted as presidential candidate at the party's Denver conference in August.
Robert Gibbs, Obama's communications director, said Obama and Clinton had spoken early yesterday and agreed to meet. "When the dust settles and it makes sense for her, he'll meet whenever she wants to," he said.
The former Democratic president Jimmy Carter said in a Guardian interview that it would be a mistake for Obama to have Clinton as his running mate.
"I think it would be the worst mistake that could be made," said Carter, who endorsed Obama on Tuesday night. "That would accumulate the negative aspects of both candidates." Lanny Davis, a close Clinton adviser and friend, was among those who yesterday launched petition drives, websites and a letter campaign directed at Obama.
Another supporter, Robert Johnson, the founder of Black Entertainment Television, said he wrote to the Congressional Black Caucus – a grouping of black members of Congress – asking members to push Obama to choose Clinton. He said he had spoken with her on Tuesday and she was "absolutely ready" for the job.
Over the last few months, even when it was obvious the numbers were against her, Clinton had refused to speak publicly about becoming vice-president. The first inkling came on Tuesday when she spoke of her interest to members of Congress who support her. This firmed up after the polls closed on Tuesday and by yesterday it had become a full-blown campaign.
Obama and Clinton met yesterday in Washington on the sidelines of the annual conference of the pro-Israeli lobby group Aipac. Obama said afterwards that there was no detailed discussion. "We're going to be having a conversation in the coming week," he said.
A Clinton aide said it would be a major problem for Obama not to put her on the ticket because many of the 18 million people who voted for her would not necessarily vote for him, including Hispanics, white working people and women.
The Obama team hopes the pro-Clinton pressure will die away over the next few days and allow him to focus on McCain.
The Republican candidate yesterday congratulated Obama on his victory and challenged him to a town hall debate every week between now and the Democratic convention at the end of August.
McCain, speaking at Baton Rouge, Louisiana, said John F Kennedy had made such an agreement with Barry Goldwater for the 1964 election.
"I don't think we need any big media-run production, no process question from reporters, no spin rooms," McCain said. "Just two Americans running for office in the greatest nation on Earth, responding to the questions of the people whose trust we must earn."
He suggested the first such debate be held on June 12 in New York.
David Plouffe, Obama's campaign manager, said such meetings would allow "a great conversation to take place about the need to change the direction of this country". The two teams are to discuss the structure and timetable.