Despite a crowded Democratic primary field and a flurry of high stakes primary races scheduled for February, major labor unions have have been unusually cautious to endorse any candidates, taking their time to vet over 20 contenders. But on Monday, United Electrical, Radio, and Machine Workers of America (UE), a 35,000-worker industrial union, announced it is throwing its support behind Vermont Senator Bernie Sanders.

The endorsement arrives just one day after Sanders rallied with striking AT&T workers in downtown Louisville, Kentucky, and less than a week after Sanders released his plan for Workplace Democracy, which pledges to double union membership in the United States by the end of his first term in 2025 with the expansion workers’ rights to unionize, strike, and bargain.

“Bernie has consistently come to UE strike lines and workplace actions,” a press release announcing the endorsement from UE reads. “He has assisted in contract fights.” Both the union and Sanders are known for their progressive stances on a number of issues, including support for Medicare For All. Welders, tool and die makers, sheet metal workers, truck drivers, warehouse workers, and custodians make up the union’s rank and file.

In February, Sanders joined a picket line of 1,700 striking UE workers outside a locomotive plant in Erie, Pennsylvania for the first major manufacturing strike of the Trump era. Sanders’ “support for UE members during the nine-day strike gave us needed national recognition, and was instrumental in achieving a just settlement,” the UE endorsement reads. At the time, Sanders used his massive email list to push for national support for the workers.

When the multinational firm Wabtec acquired the GE-owned manufacturing facility in Erie earlier this year and refused to honor its preexisting union contract, workers overwhelmingly authorized a strike. In his plan for workplace democracy, Sanders promises to force companies that merge “to honor existing union contracts,” making reference to the recent UE strike. (Eerie was one of three counties in Pennsylvania to elect Trump in 2016 after supporting Obama in 2012.)

The union voted to back Sanders after he spoke on Monday at its national convention in Pittsburgh. “The UE is about what our campaign is about,” said Mr. Sanders in a speech. “It’s time that the working class got on the offensive.”