(Reuters) - The U.S. postal workers union's executive board said on Thursday it had voted to endorse Bernie Sanders for the Democratic presidential nomination in 2016, the union said in a statement. The endorsement was a setback for Democratic frontrunner Hillary Clinton, who has been vying with Sanders, her chief rival, for the support of organized labor."Politics as usual has not worked. It's time for a political revolution," said Mark Dimondstein, president of the postal workers union. Dimondstein called Sanders, a U.S. senator from Vermont, "a true champion" of workers.