UPDATE: Dan Froomkin weighs in:
No one would deny that one of the duties of the president of the United States is to place people of his choosing in key positions throughout the executive branch, including in key law-enforcement positions. [...]
But in law-enforcement jobs — such as the attorney general, the director of the FBI, and the country’s 93 U.S. attorneys — overtly partisan behavior is a more troubling problem. While the men and women in those positions serve at the pleasure of the president, it is also a critically important part of their job to remain independent.
That’s because it’s flatly un-American for the law to be used as a political weapon. It erodes public confidence in the justice system, and offends the American commitment to fairness. It’s the sort of thing that, quite properly, can lead to impeachment.