LANSING — A conservative group sued Wednesday to stop Michigan State University from offering health insurance to the partners of gay workers and said the school is violating a 2004 amendment to the state constitution.

The American Family Association of Michigan filed the lawsuit in Ingham County Circuit Court and hopes to get a ruling setting a precedent that would block domestic-partner benefits at other state universities.

The purpose of the suit is to ensure that courts rule on the constitutionality of domestic-partner benefits at public universities, said Patrick Gillen, an attorney for the Thomas More Law Center in Ann Arbor, which is representing the association.

By providing same-sex benefits, MSU is “recognizing same-sex marriage in substance, if not by label,” Gillen said.

Deborah Labelle, an attorney for the American Civil Liberties Union of Michigan, said the suit is pointless because the state appeals court already is set to rule on the issue. The court heard arguments in a related case in April.

She characterized attempts to strip benefits from gay partners as “mean-spirited.”

“In one fell swoop, they manage to diminish both what marriage is and to exaggerate the impact of domestic-partner benefits,” she said.

MSU spokesman Terry Denbow said the school would not comment on pending litigation.

State schools that provide benefits to gay couples include the University of Michigan and Eastern Michigan, Central Michigan, Northern Michigan, Wayne State, Saginaw Valley and Oakland universities.