Black superdelegates who support Hillary Clinton are facing a crescendo of racially tinged political pressure to switch their allegiance to Barack Obama.

Rep. Diane Watson, D-Calif., said she has received several e-mails from individuals within her congressional district saying they won't vote for her if she doesn't change her support to Obama, and she should vote for him because she, too, is black. Watson said she also is getting e-mails from around the country.

And just Friday, she said, "I was told today that they're building up opposition," possibly fielding a Democratic opponent for her November election.

The efforts, while they pale in comparison to death threats she received years ago when she supported school busing, are unseemly: "I don't intimidate easily, and I don't think that's the way a democracy works," Watson said.

"I'm quite upset. ... People have stooped to a new low," she said, speaking with FOXNews.com

"To be threatened with opposition because I don't support one candidate, that's unusual in democratic politics," Watson added.

Rep. Emanuel Cleaver II, D-Mo., also a Clinton supporter, said he has not personally received threats, but he has attended meetings with other black superdelegates who he said are being harassed by Obama supporters.

He told Politico.com that his colleagues are receiving "nasty letters, phone calls, threats they'll get an opponent, being called an Uncle Tom" -- the racial pejorative used to describe blacks who are subservient to whites.

Speaking with FOXNews.com, he said, "I am saying that there are people out here in the world who have gotten so caught up on this election that they are in fact willing to harass and threaten people who are not falling in line."

He doesn't believe Obama is behind the efforts, but nevertheless, "there are some [threats] that have gone too far."

The pressure has increased in recent days after Georgia Rep. John Lewis, a black civil rights icon, switched his support from Clinton to Obama.

In recent weeks as the race for unpledged delegates has become more central to the nomination for the presidency, Obama supporters have begun calling on superdelegates -- the 795 members of Congress, party officials and others who can cast their vote any way they like at the August convention -- to cast their convention vote the way voters in their districts voted.

One such Web site, ColorofChange.org specifically targets members of the Congressional Black Caucus, calling on its viewers to "Tell CBC superdelegates to uphold the will of the voters" on its front page, and provides a form letter that readers can fill out and send to their congressmen. The letter does not contain any overt political threats, but it does ask the lawmakers to follow voting trends in their home districts.

Of the 43 CBC members, 15 have indicated their support for Clinton and 23 have indicated their support for Obama. Five remain uncommitted.

"I'm writing to ask that you use your power as a superdelegate to amplify the voice of the informed, engaged, and diverse electorate in your district and across Black America, not silence it. I urge you to make it clear that as a superdelegate, you will support the voters' will," the ColorofChange.org form letter reads.

In Cleaver's case, that would mean that he would vote for Obama. His district favored Obama 57 percent to Clinton's 43 percent.

But Cleaver disputes that argument, saying it's flawed reasoning to just go the way of his congressional district. For one thing, he said he also had Republican and independent voters who supported him in the last election, and his constituency is 24 percent African American. He said it would be impossible to explain to his white constituents that he switched his vote to Obama just because he is black.

But also, Cleaver said the superdelegate system was designed to separate the superdelegates from the primaries, which means he's not changing his mind.

"I've never even almost moved from my position, so I'm clearly a Clinton supporter," Cleaver said.

And neither is Watson.

Watson said she's worked with Clinton on universal health care, and she believes Clinton is accessible and will listen. But she said she thought Clinton's leadership following the Sept. 11, 2001, terror attacks solidified her support.

"I thought, 'There's our next president," Watson said.

FOXNews.com Greg Simmons and the FOX Brainroom contributed to this report.