United Auto Workers president Dennis Williams told reporters in May that his group is watching Tesla "very closely," interested in unionizing Tesla's Fremont, Calif., assembly plant that remains the only American-owned plant in the U.S. that isn't represented by a union.

CtW Investment works with pension funds sponsored by unions affiliated with the Change to Win federation, a group not affiliated with the AFL-CIO or UAW that includes the International Brotherhood of Teamsters, Service Employees International Union and United Farm Workers.

The group has a history of activism, engaging in recent years with a range of companies including Walmart, McDonald's, Dow Chemical, Aetna and JPMorgan in addition to Tesla.

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