HARDBALL IN THE HOMESTRETCH…. Late last week, we learned the Service Employees International Union, one of the nation’s largest and most influential labor unions, was beginning to urge wavering Democratic lawmakers to support health care reform. A top SEIU official in New York told Rep Mike McMahon (D-N.Y.), who opposed reform in November and is likely to do so again, “If you can’t support this, we can’t support you.”

It’s a reminder that when it comes to getting reform done, progressives are relying on both carrots and sticks. Take MoveOn’s latest push, for example.

In a warning shot to wavering Democrats, the progressive action group MoveOn.org is making a major push to raise money on behalf of primary challengers to those House lawmakers who vote against health care reform.

The group is set to blast out an email to its five million member list Monday asking recipients to pledge anywhere from $25 to $200 (or more) for the purposes of defeating conservative Democrats who help defeat the legislation.

“Health care reform is in serious danger in the House of Representatives: with a handful of conservative Democrats wavering, we don’t yet have the votes to pass the final bill,” reads the email, which was sent in advance to the Huffington Post. “So we’re asking every MoveOn member: will you pledge to support progressive primary challengers to House Democrats who side with Republicans to kill health care reform?

“With the big vote happening as early as this Friday, conservative Democrats need to know the stakes if they choose to side with Big Insurance over the voters on health care reform,” the email goes on. “Our pledge will send that message loud and clear. We’ll publicize the amount pledged, and make sure the media and every wavering representative know about it.”

The initiative comes just days after 83% of MoveOn members expressed their support for the Democratic health reform proposal.

For some House Dems, the threat may seem hollow — filing deadlines have already passed, so it’s too late for a primary challenger to take them on. But there are still plenty of Dems who may yet draw a primary opponent, and for the rest, there’s always 2012. That may seem like a long ways off, but for Dems who kill this once-in-a-generation opportunity, the betrayal will not soon be forgotten.

Update: In related news, New York’s Working Families Party, “a major state player whose ballot line is a prized asset to Democratic candidates, will deny its endorsement to any member of Congress who votes ‘no’ on health care legislation, party officials said.” For a few wavering Dems in the Empire State, this might matter.

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.