The 130,000-member union, known formally as the International Association of Bridge, Structural, Ornamental and Reinforcing Iron Workers, said it was supporting Mr. Biden’s candidacy because of his commitment to workers and creating infrastructure jobs through his plan to upgrade the nation’s highways, roads and bridges.

Most of the nation’s largest labor unions have stayed on sidelines during the Democratic primaries, wary of wading into an unsettled race where four candidates are within striking distance of victory in Iowa. Several unions offered early endorsements in 2015 to Hillary Clinton but their leaders later came under criticism from some rank-and-file members who supported Vermont Sen. Bernie Sanders.

Organized labor is a key constituency in the Democratic primaries and union workers often serve a vital role in get-out-the-vote efforts in primary states. The campaigns have sought to connect with a slice of union workers who voted for President Trump in 2016.

In recent weeks, labor leaders have been weighing support for Mr. Biden, Mr. Sanders or Massachusetts Sen. Elizabeth Warren, but most of the union endorsements have come from smaller local unions and state affiliates. Mr. Biden was endorsed by the International Association of Fire Fighters, which has 320,000 members, within days of his presidential announcement last spring. Mr. Sanders was backed by the National Nurses United, a union of 150,000 members that also endorsed him in 2016.

Eric Dean, the Iron Workers’s general president, said in a statement that Mr. Biden was “first among the Democratic candidates in giving labor a seat at the table and focusing on infrastructure jobs to fix our crumbling bridges.”

Greg Schultz, Mr. Biden’s campaign manager, said the former vice president was “running for president to rebuild the middle class—and to him that means protect and grow unions and collective bargaining.”

The union supported Mrs. Clinton in 2016.

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