Alberto Gonzales, following the lead of Kyle Sampson, has drawn a well defined line at what would be an "improper" reason to fire a U.S. attorney: the motive of affecting a particular criminal case.
As Sen. Sheldon Whitehouse (D-RI) pointed out, that's a "very low bar." What Gonzales is describing is what would constitute possible criminal obstruction of justice. Gonzales admitted as much, saying that in considering what would be improper, he'd thought, "what is the legal standard?"
Pressing on, Whitehouse tried to get Gonzales to understand that it would also be improper to fire a U.S. attorney for a generally political reason -- that attempting to discourage U.S. attorneys who might follow in Carol Lam's path, for instance, would be improper. Gonzales didn't seem to grasp the idea.