Hillary Clinton has begun a speech in Washington in which she is expected to formally end her marathon campaign to become the first US woman president.
Mrs Clinton started by thanking staff and supporters at the end of her 16-month campaign to become the Democratic Party's nominee.
She is set to throw her full support behind former rival and primary race winner Barack Obama.
There is intense speculation about who he will choose as his running mate.
The BBC's Kevin Connolly, at the National Building Museum in Washington, says Mrs Clinton's supporters had queued there for six hours in gruelling humidity to hear her.
He says there is an angry feeling among many of her supporters that Barack Obama only won because the complex rules of the Democratic Party process suited his campaign.
However, in an online message ahead of the speech, she said it would focus on "how together we can rally the party" behind Mr Obama in the battle against Republican presumptive nominee, John McCain, in November's election.
"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," she says.
Mrs Clinton is expected to campaign for Mr Obama to help him win the voter blocs that favoured her in the primary elections. She will also seek help in tackling her $30m (£15m) campaign debt, analysts say.
Although Mrs Clinton met Mr Obama on Thursday, little has been made public of the meeting.
Mrs Clinton has distanced herself from reports that she was hoping to be on the November ticket with Mr Obama.
A statement from Mrs Clinton's campaign on Thursday said: "While Senator Clinton has made clear throughout this process that she will do whatever she can to elect a Democrat to the White House, she is not seeking the vice-presidency."
It adds: "The choice here is Senator Obama's and his alone."
Mr Obama won enough delegates to effectively secure the nomination after primaries in Montana and South Dakota on Tuesday.
Barack Obama now faces the key choice of running mate
The Illinois senator was not expected to attend Mrs Clinton's farewell event.
On Friday he made a surprise appearance at festivities in his hometown of Chicago to celebrate the city's place on the shortlist to host the 2016 Olympic Games.
Mr Obama, under pressure from Clinton backers to choose her as his running mate, has announced a team to help him make his selection and said he would not be rushed.
The BBC's Kim Ghattas in Washington says that although the decision on the running mate is Mr Obama's, Mrs Clinton would also face a tough choice if offered it.
If she does not take it, our correspondent says, it may look as if she is not doing enough for Mr Obama, but if she accepts and Mr Obama loses she may be blamed for the defeat.
Mr McCain has challenged Mr Obama to take part in debates in 10 town hall meetings before the Democratic convention in August.
The Obama team is said to be considering the invitation.