The Service Employees International Union has endorsed Rep. Mark Critz (D-Pa.) over Rep. Jason Altmire (D-Pa.), Critz announced Monday afternoon.

The union is the latest to back Critz in the primary, continuing a troubling trend for Altmire. Critz also has the United Mine Workers and United Steelworkers in his corner.

The two congressmen were thrown into the same southwestern Pennsylvania congressional district in redistricting. The district is heavy on blue-collar workers, and unions remain a powerful political force there, especially in Democratic primaries.

Many local union representatives were furious with Altmire in 2010 for his vote against Democrats' healthcare reform law after telling them he'd support it.