Hillary Clinton will concede defeat finally tomorrow, congratulate Barack Obama and endorse him for president. But her rally in Washington, where thousands of supporters are expected to gather, is also intended as one last show of strength.

In a letter yesterday to many of those who backed her over the past six months, she said: “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. I will be speaking on Saturday about how together we can rally the party behind Senator Obama.”

The delayed endorsement — which ensures that Mrs Clinton will remain the centre of attention for the rest of the week — as well as her implication that she must play a pivotal role in uniting the party, continues to grate inside Mr Obama’s camp.

Both President Bush and the Republican nominee-elect, John McCain, have already congratulated Mr Obama on his victory. Mrs Clinton has not. Instead, she declared at a rally on Tuesday night that her 18 million-strong army of voters should be shown some respect.

During much of Wednesday there were still doubts about whether she would concede defeat at all as her campaign, even in its death throes, demonstrated the dysfunction that has characterised it for much of the past 16 months. Mark Penn, her former chief strategist, who is widely blamed for her many acts of self-destructive aggression, is understood to have urged her to “make Obama grovel”.

Geoff Garin, Mr Penn’s replacement, intervened to tell her that she risked damaging her own future — as well as Democratic hopes of retaking the White House in November — if she appeared ungracious.

A series of conference calls with her backers on Capitol Hill underlined how there was little appetite left for defiance, which, she was warned, was beginning to resemble a stubborn refusal to acknowledge reality. Almost two dozen members of the House of Representatives and eight senators are said to have told her bluntly to quit. “We pledged to support her to the end,” Charles Rangel, a senior black congressman from her home state of New York, said. “Our problem is not being able to determine when the hell the end is.”

Some staff members were in tears when she visited her campaign headquarters in Arlington, Virginia, on Wednesday to tell them not to bother coming to work after today. Others were yesterday said to be drawing “a huge sigh of relief”.

Mrs Clinton waited until Wednesday evening before announcing that she would concede, planning a rally initially for today before moving the date back 24 hours so that more of her supporters could attend. Mr Obama, who had spoken to her briefly by telephone earlier in the day, learnt of the decision through the media.

Yesterday sources indicated that she was unlikely to withdraw formally from the race. Instead, her favoured option is to suspend her campaign, allowing her to maintain control over her convention delegates and to continue to speak out on issues such as universal healthcare. It would also enable her to carry on raising money to pay off an estimated $21 million (£11 million) of campaign debt.

Mrs Clinton is said to have told friends that she wants a roll-call of delegates at the Democratic convention in August so that her daughter, Chelsea, can hear it. One party strategist yesterday voiced suspicions that her real motive was that she was “addicted to drama”.

Others believe that she is seeking to use her vast support — which, by some counts, is larger than that of Mr Obama — as leverage to force her way on to the ticket as the vice-presidential nominee.

Mr Obama’s aides are extremely reluctant to countenance such a prospect, suggesting that Mrs Clinton may overshadow him or undermine his message of changing Washington. There were also reports yesterday indicating that Bill Clinton's secretive post-White House financial deals would not pass muster in a vice-presidential vetting process.

However, some of Mrs Clinton’s supporters are pushing hard to get her the Number Two slot. Congresswomen Debbie Wasserman Schultz and Stephanie Tubbs Jones were yesterday among those demanding it.

Others say such efforts are unseemly or counter-productive. Yesterday, Mrs Clinton’s spokesman said: “She is not seeking the vice-presidency, and no one speaks for her, but her. The choice here is Senator Obama’s and his alone.”

As for Mr Obama, he says: “It’s very important for me to meet with and talk to her about how we move this party forward.” After fighting so hard and so long himself, he is aware that his rival may need a few days to come to terms with defeat.

Mrs Clinton is clearly proud of the way that she carried on until the bitter end — and proud of her supporters with whom she made a genuine connection in the latter stages of the campaign. But, as tomorrow will once again demonstrate, she has never really suffered from a deficit of pride.

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