A powerful West Coast union of dock and warehouse workers has endorsed Sen. Bernie Sanders (I-Vt.) for president, giving his campaign a boost heading into a series of primary contests there.

The International Longshoremen and Warehouse Union said its executive board voted on the endorsement Thursday. Its president, Robert McEllrath, said in a statement that Sanders is "best on the issues that matter most to American workers." The union, known for its militant history, now represents roughly 50,000 workers in California, Oregon, Washington, Alaska and Hawaii.

Alaska, Hawaii and Washington will all hold Democratic caucuses on Saturday, and it's possible Sanders could sweep all three.

Craig Merrilees, an ILWU spokesman, told The Huffington Post that the rank-and-file backing for Sanders is clear.

"The support was significant at the grassroots level," Merrilees said. "Many local bodies throughout the union had already recommended endorsements."

The union plans to participate in an upcoming Sanders rally in Washington, Merrilees said.

Despite winning a handful of contests in the past week, Sanders is gradually losing any clear path to the Democratic nomination. Hillary Clinton leads him by more than 300 pledged delegates, a spread he won't be able to close without racking up the sort of large-margin victories in big states that have so far eluded him. His best hope, as Politico reported, is to win big in Washington and parlay that into a victory in delegate-rich Wisconsin in early April, thereby keeping his nomination hopes alive.

ILWU is the fifth major union to endorse Sanders for president, following the Amalgamated Transit Union, which declared its backing for him last week. More than 20 unions, however, have lined up behind Clinton, including the largest public-sector unions in the country.