The congressman's office responded by saying he is "not a rubber stamp for special interest politics":

In what seems intended as a shot across the bow of House Dems wavering on health reform, top officials with the labor powerhouse SEIU have bluntly told a Democratic member that they will pull their support for him — and will likely field a challenger against him — if he votes No on the Senate bill.

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: Mike 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,” Fishman continued.

McMahon enjoyed heavy labor backing when he was elected to his conservative Staten Island district in 2008. He voted No on the bill last time but was said to be undecided on the Senate bill, and labor had hoped to win his support for the crucial final vote.

Fishman said SEIU officials were intent on sending a message to other House Dems that they risk losing the union’s support if they don’t vote for the bill — and said the union’s rank and file membership strongly wanted reform to pass.

“We put an enormous amount of effort into electing Democrats,” Fishman said. “This is the most important issue on everyone’s plate. We’re sending a message to Democrats: If you can’t support this, we can’t support you.”

McMahon, who comes from a conservative district, has opposed health care reform in the past, saying it would “discourage entrepreneurial activity and job growth” because its burden would fall too heavily on small businesses.