TRENTON, N.J. — Sixteen Democratic New Jersey senators say it’s “inconceivable” the state’s biggest teachers’ union is backing a Republican lawmaker in a contest against the powerful Democratic Senate president.

The lawmakers wrote to the head of the New Jersey Education Association on Friday, expressing their disagreement with the union’s support for Republican Fran Grenier in this year’s 3rd Legislative District race.

Union President Marie Blistan says the union isn’t an arm of the Democratic Party.

The influential labor union is backing Grenier over Senate President Steve Sweeney, who had a falling out with the union last year after he went back on a promise to post a constitutional amendment requiring state pension payments.

The Democratic senators say the union’s support of Grenier makes it more difficult to expand the Democratic majority in the Legislature by siphoning off resources.