By Luciana Lopez and Amanda Becker

NEW YORK/WASHINGTON (Reuters) - The U.S. postal workers union on Thursday endorsed Bernie Sanders in his bid for the White House, delivering a boost to the Vermont senator's underdog campaign against Democratic front-runner Hillary Clinton.

The American Postal Workers Union, which represents more than 200,000 postal service workers and retirees, praised Sanders as a champion of workers. It is the second national union to endorse him.

"Politics as usual has not worked. It's time for a political revolution," said Mark Dimondstein, president of the union, citing efforts by Sanders to keep post offices and mail facilities open amid budget cuts.

"He doesn't just talk the talk. He walks the walk," Dimondstein said of Sanders, who has suggested that post offices could play a role in providing modest banking services such as savings accounts and check cashing in some areas.

The endorsement gives Sanders a greater foothold within the AFL-CIO, the influential labor federation that includes 56 unions. But it will not necessarily lead to backing from other unions, prized for their ability to get members to the polls.

Clinton still has a commanding lead in the race for labor support, having won endorsements from a dozen unions with about 8 million members, including AFSCME, the public employees union, and the American Federation of Teachers.

Sanders also was endorsed in August by National Nurses United, which represents about 185,000 members.

Sanders, a self-described democratic socialist, has battled perceptions that Clinton would be more electable in the November 2016 election. Clinton, the former first lady, senator from New York and secretary of state, also enjoys longstanding ties with many national labor leaders.

Labor for Bernie, a volunteer group of union activists working to nominate Sanders, has sought to slow or even halt the flow of Clinton endorsements by national unions so labor leaders have more time to evaluate Sanders.

On Thursday, several members of Service Employees International Union, or SEIU, wrote to their union chief and international executive board members asking them to hold off on presidential endorsements ahead of the Democratic primaries beginning in February.

"It would not be the end of the world to endorse Sanders, or allow locals to go their own way in the primaries, or to sit out the primaries altogether," the SEIU members wrote.





(Reporting by Caren Bohan; Editing by Bill Trott and Alan Crosby)