(Bloomberg) -- Alphabet Inc. is illegally using return-to-office policies as a tool to try to derail YouTube contract workers from organizing in Texas, a union alleged in a National Labor Relations Board complaint.

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In the filing Tuesday, the Alphabet Workers Union accused the tech giant and a staffing firm of responding to a YouTube unionization campaign by announcing new stricter rules. Under the restrictions, set for February, refusing to return to the local office in Austin will be considered “job abandonment” and “voluntary termination,” the union said.

The policy change will punish workers who have grown accustomed to doing their jobs remotely and falls especially hard on employees who live outside the area, many of whom are pro-union, the group said. The complaint also alleges that a supervisor made implied anti-union threats and that management has been transferring work to offices in other places — including India — in an attempt “to chill the union organizing effort” by the Texas staff.

Treatment of contract workers has become a flashpoint for Alphabet — the owner of Google and its YouTube unit — which relies heavily on staffing firms to meet its labor needs. Contract employees became the majority of the company’s global workforce in 2018, and Alphabet Workers Union has been trying to organize both direct and subcontracted staff.

Tuesday’s complaint was filed against both the staffing vendor managing the workers, Cognizant Technology Solutions Corp., and Alphabet itself, which the union contends is a “joint employer” — a company that exercises enough control over workers to be legally responsible for their treatment and is obligated to negotiate if they do vote to unionize.