Service Employees International Union member Sarah Swisher of Iowa withdrew her support of Clinton, the senator from New York, after the union threw its support behind Obama, the senator from Illinois, the New York Daily News reported. There are hundreds of so-called super delegates -- party insiders -- who will get to cast votes at the Democratic convention if neither candidate has captured a majority of delegates through the state-by-state nominating contests.

"There's a lot of stuff going beneath the surface, you know, stuff like, 'We'll get you if you don't support us,'" a person connected to the Congressional Black Caucus said.

Rep. David Scott, D-Ga., also switched to Obama after the state's primary election last week.

"You've got to represent the wishes of your constituency," Scott said.

Clinton supporter, Rep. John Lewis, D-Ga., also reportedly has been teetering amid Obama pressure.

"Those people in the South, where Obama is getting 80 percent of the vote, are under tremendous pressure," a Clinton supporter said.