Baltimore Mayor Martin O'Malley will pick up an early labor union endorsement Monday in Maryland's 2006 Democratic primary for governor.

The Service Employees International Union Local 1199 is set to announce its support for O'Malley, whom the group backed in 1999 when he ran for mayor. The union includes about 7,500 hospital and health care workers from Maryland, primarily in Baltimore City and Baltimore and Prince George's counties.

Quincey Gamble, the group's political director, said it gave serious consideration to endorsing O'Malley and Montgomery County Executive Douglas M. Duncan, whom union leaders met this summer.

Gamble said the union was convinced that O'Malley has the best shot at beating Gov. Robert L. Ehrlich Jr. (R) in November 2006.

"Doug Duncan was very impressive, but right now, Martin O'Malley is the man," Gamble said.

Scott Arceneaux, Duncan's campaign manager, played down the significance of the endorsement. "It's not unexpected when a Baltimore-based union that has always supported the mayor comes out and supports him now," he said.

Arceneaux said Duncan will have union support, too, particularly from the Washington area. "It's all about who makes the best use of the support," he said.

Thomas F. Schaller, a political science professor at the University of Maryland Baltimore County, said the endorsement was not surprising but is significant, particularly given how early it is.

"A primary is less about what's been said in speeches than mobilizing a constituency," said Schaller, who is active in Democratic politics and is supporting O'Malley.

Although other unions have a larger presence in Maryland, SEIU Local 1199, whose membership stretches along the Eastern Seaboard, has a reputation for political sophistication and dedication to candidates it endorses. It is one of five local SEIU unions with membership in Maryland.