A local union branch of New Hampshire state and local employees said Sunday that it had voted to endorse Senator Bernie Sanders of Vermont for the Democratic presidential nomination, another labor endorsement for a candidate whose political stock continues to rise less than a month before the Iowa caucuses.

The endorsement, which will officially be made on Monday, is particularly notable because the union chapter, SEA/SEIU Local 1984, has acted separately from its national affiliate organization, the Service Employees International Union. That group, which represents about two million workers nationally, has remained neutral in the endorsement process up to this point, balancing relationships with multiple candidates and regional concerns that differ across local chapters.

But the New Hampshire chapter wanted to use its political firepower to back Mr. Sanders in time for the state’s first-in-the-nation primary, Rich Gulla, the local president, said in a statement. About 10,000 workers across the state are represented by the union’s collective bargaining contract.

The chapter also voted to endorse Mr. Sanders during the 2016 presidential election — even as the national organization backed his rival and the eventual Democratic nominee, Hillary Clinton.