One of Speaker Robert DeLeo’s top deputies plans to resign this month, according to people familiar with the decision, a move that would also remove from House leadership a chief advocate for a casino in Revere.

Representative Kathi-Anne Reinstein, first elected to the House in 1998, plans to step down to accept a post as government affairs manager for the Boston Beer Company, said people with knowledge of her decision.

The company, which brews Sam Adams beer, has been shopping for a government affairs executive for months, eliciting interest from several prominent figures in local politics.

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Reinstein’s decision comes at a pivotal time in the ongoing debate about where to site a Massachusetts casino. The Revere Democrat has been a vocal proponent for Suffolk Downs and Mohegan Sun facility in Revere, after East Boston residents last year voted down such a venue in their neighborhood.

Her departure also takes out a top ally of DeLeo’s. Reinstein has been a staunch DeLeo backer since early in his campaign to become speaker, and serves as an assistant majority leader in the House.