WASHINGTON (CNN) -- MoveOn.org, a grassroots powerhouse that supports Sen. Barack Obama for the Democratic presidential nomination, launched a fundraising drive Thursday to counter Sen. Hillary Clinton's wealthy supporters.

A MoveOn.org campaign for Sen. Barack Obama will spotlight new and old ways of fundraising.

Her supporters have recently argued with their checkbooks that superdelegates should vote their conscience at the Democratic National Convention in August.

MoveOn's drive sets up a face-off that illustrates the widening gap in the Democratic Party between some of its traditional financial backers, many of whom support Clinton, and a Netroots donor base that leans toward Obama.

Twenty of Clinton's major donors sent a letter to House Speaker Nancy Pelosi Wednesday that suggested they might rethink their support for the party's congressional efforts this cycle if Pelosi did not alter her publicly stated view that superdelegates should support the party's pledged delegate leader -- a position that would be fatal to Clinton's presidential bid.

"We have been strong supporters of the DCCC," they wrote. "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."

The DCCC -- Democratic Congressional Campaign Committee -- assists the party's House candidates.

A day later, MoveOn.org announced its fundraising drive to demonstrate its support for Pelosi's position

"It's the worst kind of insider politics -- billionaires bullying our elected leaders into ignoring the will of the voters," wrote organizers in an e-mail to the group's members. "But when we all pool our resources, together we're stronger than the fat cats. So let's tell Nancy Pelosi that if she keeps standing up for regular Americans, thousands of us will have her back. And we can more than match whatever the CEOs and billionaires refuse to contribute."

Senior advisers to Clinton's campaign denied Thursday the campaign had anything to do with the donors' message to Pelosi.

"We got a heads up that a letter was being sent, but we didn't know what was in it and that was it. Our supporters let us know that they were sending something over," said Clinton spokesman Phil Singer on a conference call with reporters.

But he would not repudiate the content of the note.

"I think that the letter speaks for itself," said Singer. "There's clearly a broad feeling among many Democrats, many people who are active in the party, that the role of superdelegates is to exercise independent judgment, to make their decision based on what is best for the party, what is best for the country."

It's a delicate balancing act for the Clinton team. Its success will depend in part on this kind of donor pressure. But the campaign cannot be seen by the majority of the party to be endorsing these actions in any way without risking a major backlash -- a point not lost on the Obama team, which moved quickly to highlight the message's implied threat.

"This letter is inappropriate and we hope the Clinton campaign will reject the insinuation contained in it," said spokesman Bill Burton. "Regardless of the outcome of the nomination fight, Sen. Obama will continue to urge his supporters to assist Speaker Pelosi in her efforts to maintain and build a working majority in the House of Representatives."

The nonpartisan Center for Responsive Politics reported Thursday that the 20 Clinton donors who signed the letter to Pelosi had collectively contributed nearly $24 million to Democratic candidates and committees over the last decade, including more than $500,000 to Clinton's senate and presidential campaigns and leadership PAC. They have collectively donated less than a tenth of that amount to Obama.

The Pelosi letter is not the first time Democratic heavyweights backing Clinton's run have contacted party officials to press for decisions critical to her campaign and hinted their financial support might depend on the party's positions.

Earlier this month, The New York Times reported Clinton fundraisers in Florida and Michigan were threatening to withhold funds intended for the Democratic National Committee or ask for refunds of previous donations if the party did not seat their state's full delegations at the convention or hold new contests in those states.

Drawing the bulk of superdelegate support regardless of the pledged delegate outcome and seating the Michigan and Florida delegations despite party penalties have both been priorities for the Clinton campaign.

The idea that some Clinton backers might keep their checkbooks closed if she does not win the Democratic nomination comes as a Gallup Poll this week found that 28 percent of her supporters might back presumptive Republican nominee Sen. John McCain this fall if she is not on the ballot.

Pelosi and DNC Chairman Howard Dean have so far defended their positions in the face of the mounting pressure.

"The speaker believes it would do great harm to the Democratic Party if superdelegates are perceived to overturn the will of the voters," said Pelosi spokesman Brendan Daly after the release of this week's letter. "This has been her position throughout this primary season, regardless of who was ahead at any particular point in delegates or votes."

Earlier in the month, the national party seemed to take an even tougher tone against these critics.

"While Howard Dean has been working hard to be an honest broker, too many involved have been more concerned with headlines than results," DNC spokeswoman Stacie Paxton told the Times. "It's never productive to negotiate through the press, but make no mistake, Howard Dean will continue to lead the effort to find a workable solution that's fair and consistent with the rules."

The DNC, however, may be particularly vulnerable if similar threats continue. As both the party's presidential candidates continue to break fundraising records, the national committee stands as the sole Democratic entity at a cash disadvantage to its Republican counterpart, with less than $5 million cash on hand, according to the latest FEC filing -- a fifth of the RNC's war chest. E-mail to a friend

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