Mr. Rosenstein did, of course, work for the Obama administration as the U.S. attorney for Maryland, but he was appointed to that post by George W. Bush. And if he did that job with an aim toward pleasing the Democrats, it would probably be news to the officials in that party he prosecuted or for whom he caused huge political headaches. Among the elected Democrats he sent to prison was former Prince George's County Executive Jack Johnson, and the massive investigation his office oversaw into corruption at the Baltimore City Detention Center proved problematic for former Gov. Martin O'Malley, who was just about to launch a campaign for president on his reputation as a proficient manager. One of his last acts in Maryland was the indictment of members of the Baltimore Police Department's Gun Trace Task Force, another case that put the city's Democratic leadership in a bad light.