KANSAS CITY, Mo. — Missouri’s Democratic attorney general broke with his party on Monday and urged a federal judge to invalidate the central provision of the new health care law. The filing of the court brief by Attorney General Chris Koster, a onetime Republican state legislator who switched to the Democratic Party in 2007, underscores the act’s political tenuousness in a critical Midwestern swing state. Mr. Koster’s action followed months of pressure from state Republicans that he join attorneys general from other states who are challenging the constitutionality of the law. Rather than join the litigation, however, Mr. Koster chose...