FEAR FACTOR…. The American Medical Association wants congressional Democrats to vote for health care reform. So does the American Nurses Association. The American Cancer Society does, too, along with AARP, the NAACP, and progressive activist groups like MoveOn.org.

What do all of these groups have in common, aside from being right about health care policy? Congressional Dems who will decide whether reform lives or dies aren’t especially afraid of any of them.

Labor unions, however, are another matter entirely.

Dem Rep Mike McMahon of New York met yesterday with a top SEIU official and told him he’s likely to vote No, the official tells me. The official: Fishman, president of SEIU 32bj, the largest property workers union in the country, with 120,000 members in eight states.

Fishman told McMahon that the union would not support him if he voted No — and suggested the hunt for a primary or third-party challenger would follow.

“He let us know he’s not supportive of the health care plan,” Fishman says. “We’ve let him know that we can’t support somebody who doesn’t support it…. “We are going to begin talking to other unions about finding someone else for that seat.”

The message for incumbent Democrats from labor isn’t subtle: “If you can’t support this, we can’t support you.”

In McMahon’s case, the New York Democrat already voted against health care reform when it came up in November, but Dems saw him as a possible switcher. With labor applying this kind of pressure to Democrats who already opposed reform once, it seems likely the pressure will be even more intense to keep reform’s supporters on board with the proposal.

It a lot of districts, unions help provide campaign infrastructure for Democratic candidates — helping with get-out-the-vote drives, working phone banks, distributing campaign materials — and for Dems who are worried about generating support for their re-election campaigns, these threats will be hard to ignore.

Steve Benen

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.