Anthony Brown kicks off his campaign for his run for Maryland governor in May.. (Photo by Astrid Riecken For The Washington Post)

Maryland Lt. Gov. Anthony G. Brown (D) is set to pick up an endorsement of his 2014 gubernatorial campaign Monday from a key labor group, the Service Employees International Union, according to several people familiar with the planned announcement.

The sought-after nod from the SEIU’s Maryland/D.C. State Council will be the latest in a series of endorsements that have come Brown’s way from Democratic politicians and liberal interest groups in what is expected to be a highly competitive race to succeed Gov. Martin O’Malley (D).

Brown faces state Attorney General Douglas F. Gansler and Del. Heather R. Mizeur (Montgomery) in the June Democratic primary.

The SEIU’s decision to back Brown was confirmed by several people, including one person close to his campaign. The people familiar with the endorsement spoke on the condition of anonymity because they were not authorized to talk publicly about the announcement by the union or the campaign.

The SEIU’s Maryland/D.C. Council claims to represent 45,000 workers in the region in fields including health care, child care and security services. In a statement about Monday’s announcement, the union said its backing would mean hundreds of volunteers would knock on doors, make phone calls and speak out on behalf of its chosen candidate.

SEIU leaders are expected to cite Brown’s work on health care, including his efforts to implement the Affordable Care Act in Maryland, as well as his history of supporting legislation to raise the minimum wage in the state.

In 2006, as a state delegate representing Prince George’s County, Brown voted to override a veto of a minimum-wage bill that had been vetoed by former governor Robert L. Ehrlich Jr. (R).

Brown, Gansler and Mizeur have all voiced support of more recent efforts to further raise the state’s minimum wage. Gansler pledged last week that it would be his first order of business when he takes the governor’s office in 2015, if the legislature hasn’t addressed it by then.

Gansler has been largely dismissive of Brown’s endorsements, saying that voters, not “the Democratic establishment,” will determine the outcome of the race.

Gansler, who formally announced his bid for governor last week, has four stops planned Monday on Maryland’s Eastern Shore as part of an announcement tour. He is scheduled to be in Cambridge at 3 p.m., when the SEIU plans to unveil its pick for governor in Baltimore.