The International Union of Bricklayers and Allied Craftworkers (BAC), a group with about 80,000 members, said in a statement that “no presidential candidate is better qualified than Hillary Clinton to occupy the White House.”

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“There is only one candidate — Hillary Clinton — who can lead American workers to triumph in the face of today’s hostile political climate, difficult financial times, and relentless attacks against the right to organize,” the group said.

It’s the third union endorsement for Clinton, who has previously received backing from two other powerful groups: the American Federation of Teachers and the International Association of Machinists and Aerospace Workers.

The announcement came just hours before Sanders, who has emerged as the primary challenger to Clinton in the race for the Democratic presidential nomination, was scheduled to hold a conference call with “thousands of union members” to tout his “strong support from rank-and-file union members across the country.”

Last month, Sanders nabbed his first major union endorsement, receiving the backing of National Nurses United (NNU), the nation's largest organization of nurses.

NNU president RoseAnn DeMoro will join Sanders on the Wednesday night call, along with Larry Hanley, the president of the Amalgamated Transit Union, and Larry Cohen, the former president of the Communications Workers of America.

Neither of the latter two groups have endorsed yet.

Many of the big labor organizations, including the AFL-CIO, International Brotherhood of Teamsters and Service Employees International Union, remain on the sidelines, content to be wooed by the Democratic candidates for the time being.