Markey has also received the backing of the Massachusetts teachers' union, and the state AFL-CIO declined to endorse in the race, both blows to Lynch, who is counting on union backing to boost him to a win.
Lynch remains the underdog in fundraising and in the polls, and his campaign is hoping that his connections to the union community — lynch was once a union worker and leader himself — would bring him the support of that voting bloc, which is known in Massachusetts for offering considerable financial and organizational help to Democratic candidates.
While Lynch has secured the endorsements of more than 40 local unions by his own count, he's lost a few prominent ones, complicating his bid.
But the SEIU was unlikely to back Lynch; the union supported a primary opponent to the lawmaker in 2010, following Lynch's vote against President Obama's healthcare overhaul.
Conor Yunits, a spokesman for Lynch's campaign, said that the campaign was not counting on the backing of the SEIU.
"While we would have loved to have SEIU's support, we weren't expecting their endorsement. Congressman Lynch respects their decision and will continue working on issues that are important to their members," he said.
Lynch and Markey will face off in the Democratic primary on April 30, and the Democratic nominee is likely to win the Senate seat vacated by Secretary of State John Kerry (D). Three Republican candidates are vying for their party's nomination.