The battle over the Democratic presidential nomination turned nasty on Thursday, one day after Hillary Clinton donors subtly threatened to stop the spigots for House Democrats if Speaker Nancy Pelosi insists superdelegates vote the same way as pledged delegates.

Liberal group MoveOn.org -- which has endorsed Barack Obama -- issued a letter to its members, asking them to sign on to a statement that says millionaire donors shouldn't dictate how the race is won. It also asked for members to match the money the donors would otherwise supply.

"The Democratic nomination should be decided by the voters -- not by superdelegates or party high-rollers. We've given money -- and time -- to progressive candidates and causes, and we'll support Speaker Pelosi and others who stand up for democracy in the Democratic Party," reads the statement.

In sending the request to join, MoveOn's political action team called the donors' letter to Pelosi "the worst kind of insider politics -- billionaires bullying our elected leaders into ignoring the will of the voters."

In the letter sent to Pelosi on Wednesday, the Clinton supporters asked that the House speaker retract statements she made a week prior calling for superdelegates to follow the will of primary and caucus voters.

Superdelegates "must look to not one criterion but to the full panoply of factors that will help them assess who will be the party's strongest nominee in the general election," the donors wrote.

The letter also noted the donors "have been strong supporters" of the House Democrats' fundraising arm.

"We therefore urge you to clarify your position on superdelegates and reflect in your comments a more open view to the optional independent actions of each of the delegates at the national convention in August."

Spokesman Brendan Daly said Pelosi is confident superdelegates will make their choice between Clinton and Obama "based on many considerations, including respecting the decisions of millions of Americans who have voted in primaries and participated in caucuses. The speaker believes it would do great harm to the Democratic Party if superdelegates are perceived to overturn the will of the voters. This has been her position throughout this primary season, regardless of who was ahead at any particular point in delegates or votes."

Clinton campaign officials said on a conference call Thursday that they knew about the pointed letter to Pelosi beforehand but only in general terms.

"We got a heads up," said a spokesman, adding that everyone recognizes that superdelegates have the responsibility to exercise independent judgment about who is best for the party and the country.

The officials denied knowing the details or approving of any of the language. Pressed on the tenor of the letter, Clinton Communications Director Howard Wolfson said the Clinton camp is committed to electing Democrats to the House of Representatives and "believe strongly in Pelosi's leadership."

According to the Center for Responsive Politics, the Democratic donors who wrote Pelosi have contributed nearly $24 million to Democratic candidates and committees over the last 10 years.

Hillary Clinton's Senate and presidential campaigns and her political action committee received $554,000. By comparison, Barack Obama collected $52,200. Contributions for the Democratic Congressional Campaign Committee, which helps Democratic House candidates, equaled $2.9 million.

BET founder Robert Johnson, a Clinton supporter who has contributed $2 million to Democratic candidates over the years, and signed onto the letter, said he will continue to contribute to the party but it's the superdelegates' prerogative to choose who they want.

"The real issue is fundamental fairness in making sure that the superdelegates carry out their role. They are not, as Nancy Pelosi, would suggest 'confirmation delegates.' They are not robot delegates. They are superdelegates whose charter authorizes them to analyze who will be the better candidate of the two to run against the Republican candidate and win the presidency," Johnson said. He added that he has a letter on his desk from Pelosi asking him to contribute $100,000 to the national convention, but he has not decided yet whether he will donate.

The MoveOn team also calls for the members to match "whatever the CEOs and billionaires refuse to contribute."

Calling Clinton's donors "the old guard," the MoveOn group said, "This is how the Democratic Party used to function -- the big donors called the shots. But the small donor revolution has changed that. The 20 people who signed this letter have given Democrats an average of $2.4 million per year over the last 10 years. Small donations now dwarf that: In February alone, Obama and Clinton raised $47 million in small donations."

Click here to see the donors' list of contributions.