A statement from Clinton's campaign on Thursday said that she was "not seeking the vice-presidency", but later said the "choice here is Senator Obama's and his alone".
Many analysts say Clinton's endorsement of Obama is a necessary first step to unifying the Democratic Party.
Jennifer Palmieri, of the Centre for American Progress and former communications advisor in the Clinton White House, said: "If the country has negative feelings about the two Clintons, it's not going to help Obama. So it is in his interest to the degree to which they need rehabilitation for him to want to be a part of it."
Party strategists said she needs to set aside any bad feelings and put on a convincing show of unity.
Doug Schoen, a Democratic strategist, who worked in the Bill Clinton administration, said: "Feelings on both sides have been pretty heated and a healing process has to go on and it has to begin tomorrow."
The 16-month campaign for the Democratic presidential nomination ended on Tuesday, but Clinton raised eyebrows by not immediately conceding defeat.
The extent of Clinton's endorsement will be of keen interest to the Obama camp.
She gained more than 17 million votes during the Democratic battle, and Obama will need many of those to defeat John McCain, the presumptive Republican party presidential candidate, in November.
|Obama won enough delegates to secure the |
party's nomination on Tuesday [AFP]
The Obama campaign sought on Friday to dispel any talk that there would be a decision soon on who Obama, a 46-year-old Illinois senator, would pick as his running mate.
"It's important that this be done in a careful, methodical way. We're not going to be rushed into making any pick, whomever that might be," Obama's communications director, Robert Gibbs, said.
Democratic strategists expect Clinton to speak unequivocally about Democrats' need to rally around Obama in November.
Stephen Elmendorf, a Democratic party strategist, said he had no doubt that Clinton was "going to say all the right things", because Democrats need to avoid two examples from the past.
In 1976, the primary campaign of Ronald Reagan against incumbent Gerald Ford weakened the latter, and in 1980, Democrat Edward Kennedy's primary campaign, similarly weakened Jimmy Carter's presidential re-election drive.
Some have suggested that Clinton, 60, follow the lead of Kennedy, who returned to the senate and became one of its most effective legislators, helping craft landmark bills, many to help the disadvantaged.
"I've got to believe, as trite as it may sound, that her role model now is Ted Kennedy," Norm Ornstein, a congressional scholar at the American Enterprise Institute, said.
The New York Democrat "could become a real go-to person in the senate who has the ability to attract and keep great staff, who has a real grasp for important issues, who can build relationships and has a driving desire to figure out how to make things happen", he said.
Paul Light of the Centre for the Study of Congress said whatever decision Clinton makes, she must make it quickly or face party backlash.
"If she withdraws gracefully, she will have an enormous range of opportunities," Light said. "She could eventually become a Senate leader or end up on the Supreme Court."
Clinton has won many admirers in the Senate, among Democrats and Republicans while working with them to draft bills on matters from national security to health care, her signature issue.
In doing so, she has become one of America's most powerful women.
Clinton joined the senate in January 2001 after eight years in the White House as a top aide and wife of Bill Clinton.
Elmendorf said the "best thing for her to do is endorse [Obama] now, endorse him enthusiastically and endorse him without any precondition".