DES MOINES, Iowa (AP) — Bernie Sanders was endorsed Thursday by the 200,000-member American Postal Workers Union, an influential group that also backed the Vermont senator’s presidential bid against Hillary Clinton during the 2016 Democratic presidential primary.

The union’s support is key because it promises organizing muscle across the country. Sanders says that if turnout is high during Monday’s lead-off Iowa caucus, he will win — and a win there will key victories in the next two states that vote, New Hampshire and Nevada.

“As with 2016, once again the Sanders campaign is boldly uplifting the goals and aspirations of workers,” union president Mark Dimondstein said in a statement. “Simply put, we believe it is in the best interests of all postal workers, our job security and our union to support and elect Bernie Sanders for president.”

Polls in Iowa and other states show Sanders bunched near the top of the polls with former Vice President Joe Biden, Massachusetts Sen. Elizabeth Warren and Pete Buttigieg, the former mayor of South Bend, Indiana.