A civil war is threatening to erupt within the Democratic Party as liberal advocacy group MoveOn.org looks to punish moderate Democrats opposed to the sweeping health care overhaul proposed by party leaders.

MoveOn has reportedly raised more than $3.5 million  in contributions to fund primary challenges against "any Democratic senator who blocks an up-or-down vote on health care reform with a public option," according to an e-mail sent to group members on Thursday.

The e-mail warned that any Democratic House member who joins Republicans to filibuster the health care reform measure will "face an enormous backlash from the grassroots."

The group also highlights a letter from Democracy for America, the nation's largest progressive political action founded by former Democratic National Committee Chairman Howard Dean, which urges Senate Democrats to strip committee chairmanship from "any Democrat who filibusters health care."

MoveOn.org, which has 5 million members, has become increasingly vocal in its threats against conservative and moderate Democrats who remain opposed to the health care bills laid out by House Speaker Nancy Pelosi and Senate Majority Leader Harry Reid.

Several Democrats, including Rep. Ike Skelton of Missouri, chairman of the Armed Services Committee, and Rep. Artur Davis of Alabama, have expressed resistance to Pelosi's $1.055 trillion measure in recent days -- underscoring the intra-party rift over President Obama's top domestic priority.

Obama had planned a rare visit to the House Friday morning to persuade wavering Democrats, but was forced to reschedule it until Saturday as delays push back a vote on the measure.

Among the most heated disagreements among rank-and-file Democratic lawmakers are issues related to abortion and illegal immigrants.

Democratic opponents of abortion -- under pressure from Catholic bishops -- want stronger provisions written in the bill that no federal funds would be used to finance abortion in coverage bought in the government-run exchange.

The Associated Press contributed to this report.