The Michigan attorney general said Saturday that he plans to represent city retirees in the Detroit bankruptcy case because their pensions are protected by the state constitution.

The announcement from Attorney General Bill Schuette could set up a legal battle with Michigan Gov. Rick Snyder and Detroit emergency manager Kevyn Orr, whom the governor appointed. Messrs. Snyder and Orr have argued that cuts to the unfunded portion of city pensions are necessary to help restructure more than $18 billion in long-term liabilities...