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In the latest setback for Rep. Stephen Lynch's, D-Mass., Senate campaign, the Massachusetts Service Employees International Union on Thursday endorsed Lynch's Democratic primary opponent, Rep. Ed Markey.

While Markey entered the race to replace Secretary of State John Kerry with the backing of national Democrats and progressive activists, Lynch, a former ironworker and union president, had hoped to rely on support from the Bay State labor community. While he has the backing of more labor unions overall, Lynch has failed to win endorsements from some of the state's largest unions. The Massachusetts Teachers Union is also backing Markey, while the state AFL-CIO is not making an endorsement in the primary.

Lynch has a long history of union support, but he alienated some labor leaders with his 2010 vote against the Affordable Care Act. Later that year, then-SEIU political director Mac D'Alessandro challenged Lynch in the Democratic primary, but the congressman prevailed and won reelection.

The SEIU endorsement isn't the first disappointing news for Lynch this week. The South Boston congressman also had hoped to have the support of Boston Mayor Tom Menino, but the powerful Democrat said on Tuesday that he won't get involved in primary.

Markey has built up a significant lead in the polls. A UMass Lowell-Boston Herald survey conducted earlier this month had Markey leading Lynch 50 percent to 21 percent. The winner of the April 30 Democratic primary is expected to easily win the general election.