US Democratic presidential candidate and Senator Hillary Clinton (D-NY) speaks during a campaign visit at Maysville High School in Maysville, Kentucky, May 19, 2008. (Xinhua/Reuters File Photo) 

Obama aides expect more party "superdelegates" to jump into the Illinois senator's wagon in the next few days and give him enough votes to get the nomination.

Robert Gibbs, a spokesman for the Obama campaign, said the senator could secure the nomination as early as Tuesday, after the Montana and South Dakota contests.

"If not Tuesday, I think it will be fairly soon," Gibbs said.

But the prolonged nomination contest has alienated the voters from both sides, which is seen as a dangerous sign for the party, which aspires to take back the White House after eight years of Republican rule.

As a result, how Obama and Clinton interact with each other in the post-primary period will have a great impact on the Democrats' chance in the November general election.