The union representing postal workers is announcing its endorsement of Sen. Bernie Sanders for president on Thursday, giving the underdog candidate a much-needed boost in his effort to topple Hillary Clinton, who remains far ahead in the race for union support.

Mr. Sanders’s views and record typically line up more closely with unions’ priorities, and the American Postal Workers Union said his years of work on their causes won members over.

“Politics as usual has not worked. It’s time for a political revolution,” union president Mark Dimondstein said in a statement, echoing Mr. Sanders’s battle cry.