Gubernatorial candidate Douglas M. Duncan scored a prize yesterday in the battle for support from Maryland labor unions, winning endorsements from three chapters of the Service Employees International Union.

For the past month, Duncan (D), Montgomery's county executive, and his opponent for the Democratic nomination for governor, Baltimore Mayor Martin J. O'Malley, have been jockeying for support from Maryland's organized labor and elected officials.

But the contest between O'Malley and Duncan has split Maryland's five SEIU chapters, which represent workers in hotels, restaurant, stores and health care services.

Last month, O'Malley was endorsed by 1199 SEIU, which represents 7,500 workers, primarily in Baltimore and Prince George's counties and the city of Baltimore. Some of those members will be canvassing Prince George's neighborhoods on O'Malley's behalf this weekend.

But yesterday, three other SEIU chapters -- Local 400, whose members work in Prince George's County; Local 500, a statewide union; and District 82, a multi-state union -- endorsed Duncan. Combined, the three unions represent 20,000 employees in Maryland.

"Duncan has proven himself to be a trusted, valued member," said Merle Cuttitta, president of SEIU Local 500, based in Montgomery.

Last spring, the union was instrumental in winning General Assembly approval for legislation requiring Wal-Mart to pay more for employee health benefits or contribute to a statewide fund. But Republican Gov. Robert L. Ehrlich Jr. vetoed the bill.

Quincey Gamble, political director for 1199 SEIU, said that's one reason the split among SEIU members won't last past the primary.

"I wish we were all in the same page, but we are united in the task of defeating Bob Ehrlich," Gamble said.

SEIU Local 722, which is based in the District but has about 1,000 members in Prince George's County, has yet to endorse anyone.