A trial challenging the ban began Tuesday in Detroit federal court. In Washington, U.S. Attorney General Eric Holder told state attorneys they're not obligated to defend laws that discriminate against people based on sexual orientation.
Holder gave a speech Tuesday and cited his own experience in refusing to defend the federal Defense of Marriage Act.
Voters added Michigan's gay-marriage ban to the state constitution in 2004. Schuette spokeswoman Joy Yearout says his office will continue to "aggressively defend" it.
Schuette is a Republican. Democratic attorneys general in six states have declined to defend same-sex marriage bans that have been challenged by gay couples.