Massachusetts Attorney General Martha Coakley has filed suit (pdf) against the US government, challenging Section 3 of the federal Defense of Marriage Act (DOMA). The suit claims that the law, which defines marriage as a union between one man and one woman, unfairly excludes more than 16,000 Massachusetts married same-sex couples and their families from critically important rights and protections based on marital status. The complaint alleges that DOMA, which affects more than 1,100 federal statutory provisions, violates the United States Constitution by interfering with the Commonwealth’s sovereign authority to define and regulate the marital status of its residents. The complaint also alleges that DOMA exceeds Congress’s authority under the Spending Clause because Congress does not have a valid reason for requiring Massachusetts to treat married same-sex couples differently from all other married couples.