2020 Democratic U.S. presidential candidate and U.S. Senator Bernie Sanders speaks during the Presidential Gun Sense Forum in Des Moines, Iowa, August 10, 2019.

Sen. Bernie Sanders notched a big national labor endorsement Thursday only days before the 2020 Democratic primary nominating contests start.

The more than 200,000-member American Postal Workers Union backed the Vermont senator, the latest in a growing list of U.S. labor unions to support a Democratic presidential candidate. The endorsement comes only four days before the first-in-the-nation Iowa caucuses.

"Bernie Sanders has proven he is a fierce advocate on the side of postal workers. He has opposed the closures of postal facilities and reduced service standards," said Mark Dimondstein, president of the APWU, which represents mostly U.S. Postal Service employees and retirees.

In a statement Thursday, Sanders said he was "honored" to get the group's endorsement. He criticized the Trump administration's reported efforts to privatize the U.S. Postal Service, saying that "when we defeat Donald Trump, we're not going to privatize and cut the Postal Service. We're going to expand and strengthen the Postal Service and speed up the delivery of mail."

While only a handful of national unions have picked favorites in the 12-person Democratic field, more organizations have started to come off the sidelines as voters get closer to choosing the nominee. So far, unions — a key Democratic constituency — have showed a preference for Sanders and former Vice President Joe Biden.