By WILL WEISSERT, Associated Press

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 for Monday's leadoff Iowa caucuses, he will win — and he believes a win there will propel 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.

Warren and Sanders have long called for expanding the services offered by post offices, especially in rural communities, to include things like banking.

National labor unions wield a great deal of influence in the Democratic primary, though many have yet to pick sides in the still-crowded 2020 field. The National Nurses United backed Sanders in November after endorsing him in 2016. Biden's campaign got a boost last year with the endorsement of the International Association of Fire Fighters.

Buttigieg once worked at the high-powered consulting firm McKinsey & Company and has previously released a client list that included the U.S. Postal Service. In 2010, the Postal Service hired McKinsey and other consulting firms, and they eventually recommended ways to increase revenue, including by cutting back on daily mail service.

Buttigieg's campaign released a statement in December saying he was “part of a team tasked with generating ideas to increase revenue like selling greeting cards and increasing the use of flat rate boxes.” It added that the candidate “never worked on cost-cutting or anything involving staff reorganization or the privatization of essential post office services.”

In its statement endorsing Sanders on Thursday, the postal workers union said that the Trump administration “has released a series of proposals that would end the universal service requirement and also would make significant changes in the pricing structure of mail products." It also noted that there have been calls to sell the federally run postal service to private corporations.

“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,” Sanders tweeted in reaction to the endorsement.

The union promised to “encourage its members and their families who live and work in all 50 states" to join Sanders rallies and volunteer for campaign activities. It also said it would launch voter registration drives and urge its members, their families and their friends to choose “vote by mail” options in states without restrictions on absentee balloting.

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