Barack Obama was expected to set his sights on John McCain on Tuesday night at his victory rally in St Paul, Minnesota, the site of the Republican national convention in September.

But the Illinois senator and presumptive Democratic nominee faces a task more daunting than beating his Republican rival in the immediate aftermath of his expected victory over Hillary Clinton: persuading millions of Democrats who voted for the former first lady and staunchly support her to rally behind his bid for the White House.

After a bruising primary battle, many agree that no single person will be more crucial in uniting the Democratic party behind Mr Obama than Mrs Clinton herself, particularly among three groups of voters he needs: older women, Hispanics and blue-collar white voters. Among those groups, Hispanics represent the toughest challenge because of Mr McCain's reputation for being a sympathetic supporter of immigration reform.

"He needs her to win the people over because he needs the endorsement of someone they trust - someone who will say 'Reverend Wright [Mr Obama's controversial former pastor] is not the issue, it's the economy'," said Hank Sheinkopf, an unaffiliated Democratic consultant who previously worked for Bill Clinton. "Whether he likes it or not, they are now linked."

While big donors, Democrats on Capitol Hill and other influential Clinton supporters are expected to eventually fall in line behind Mr Obama's White House bid, people who have supported Mrs Clinton's nomination say the Illinois senator will be pressed to select her as his vice-president as a condition of their support.

"In the event that she is not the nominee, the most immediate way to keep people intensely engaged is to have her on the ticket," said Gordon Giffin, a Clinton donor.

However, not everyone agrees Mr Obama needs to rely on Mrs Clinton for victory. Mark Schmitt, a senior fellow at the New America Foundation, said it was not clear her political endorsement would sway voters.

"Obviously those are voters he needs to reach, but it is an open question whether they are reached by her vouching for him, or by him reaching them [with his own message]," Mr Schmitt said.

Still, for some Democratic insiders Mr Obama's success or failure could depend on the sincerity of a single, inevitable image from the convention in Denver this summer: that of the Illinois senator and his wife Michelle standing on the stage, clasping hands with Hillary and Bill Clinton.

"I think her role in the convention is much more important than what they do in the next 10 days. The voters will look for cues from her," said another Clinton supporter.