While some on the right have been trying to spin the USA firings as trivial, because “Clinton did it, too,” those on the right who actually did the firings had a different view: View image. (Sorry, the email document dump is all in PDF format — this is from page 20.)
UPDATE: Transcription of email from Kyle Sampson to Harriet Miers, 1/9/06:
In recent memory, during the Reagan and Clinton administrations, Presidents Reagan and Clinton did not seek to remove and replace U.S. Attorneys they had appointed whose four-year terms had expired, but instead permitted such U.S. Attorneys to serve indefinitely.
Emphasis in the original. Rove and others keep pointing to the firing of USAs that happened at the beginning of the Clinton administration (and Reagan administration), but as this email shows, Sampson himself saw that the correct point of comparison was to second term changes.