In a brief filed Tuesday, Attorney General Lisa Madigan said the $7.9 billion deal would raise rates for customers of Exelon's ComEd utility, citing testimony by the company's executives during regulatory hearings before the Maryland Public Service Commission.
The Illinois attorney general's office is among several groups that have objected to the deal on the grounds that the combined company would have two much control of electricity prices on the grid that serves much of the mid-Atlantic region.
The Maryland Office of People's Counsel and the Pennsylvania Office of Consumer Advocate have also asked the Federal Energy Regulatory Commission to hold a hearing on the issue.
FERC is reviewing the proposed merger, though a decision may not come until April — later than what the two companies had expected.
Exelon and Constellation have asked federal regulators to act on the merger by Jan. 5, which is the deadline for the Maryland PSC to make a decision.