Among the superdelegates, many former Clinton supporters are switching to Obama.

Even in national polls, he is expanding his lead against Clinton.

The latest Gallup poll shows Obama is currently favored by 55 percent of Democrats and Democratic-leaning independents, to Clinton's 39 percent.

He also has a huge advantage in terms of campaign finance.

Obama's campaign reported Tuesday the Illinois senator raised more than 31 million U.S. dollars in April, with 37 million dollars of cash on hand.

That is in sharp contrast to the 21-million-dollar debt of the Clinton campaign, which is unwilling disclose how much cash is on hand.

Challenge to unite the party

However, Obama still needs to reach out to Clinton supporters to unite the party as quickly as possible before taking on his Republican opponent.

The prolonged contest between the two candidates has caused a rift in the party that will take time to close, as exit polls from recent primaries shows.

In Kentucky, two-thirds of Clinton supporters said they would vote for Republicans or not vote at all in the general election if Clinton is not their party's nominee.

Of those, 41 percent said they would vote for Republican candidate McCain and 23 percent said they would not vote at all.

Just 33 percent said they would back Obama in the general election.