NEW YORK--(BUSINESS WIRE)--Fitch Ratings considers the District of Columbia Public Service Commission's (PSC) denial of the proposed merger between Exelon Corp. (EXC) and Pepco Holdings, Inc. (PHI; 'BBB' IDR, Outlook Stable) to be negative for PHI and its operating subsidiaries. Each of the operating subsidiaries deferred rate case filings during the 16 months the merger was under review which has and will weaken credit metrics, which were only marginally supportive of the existing ratings befo