Are we willing to accept resignation?
I am hopeful that Congress forces us to ask this question soon.
<snip>
Throughout the Congress' two hundred years, several major questions have dogged impeachment proceedings. One concerns resignations. In general, the resignation of an official puts an end to impeachment proceedings because the primary objective, removal from office, has been accomplished. This was the case in the impeachment proceedings begun in 1974 against President Richard Nixon. However, resignation has not always been a foolproof way to preclude impeachment, as Secretary of War William Belknap found out in 1876. Belknap, tipped off in advance that a House committee had unearthed information implicating him in the acceptance of bribes in return for lucrative Indian trading posts, rushed to the White House and tearfully begged President Ulysses Grant to accept his resignation at ten o'clock on the morning of March 2, 1876. Around three o'clock that afternoon, representatives, furious at both the president and Belknap for thwarting them, impeached Belknap by voice vote anyway. The Senate debated the question of its jurisdiction, in light of Belknap's resignation, and decided by a vote of 37 to 29 that he could be impeached. But at the end of Belknap's sensational trial in the summer of 1876, he was found not guilty of the charges, not because the senators believed him innocent (most did not), but because most had decided they in fact had no jurisdiction over Belknap, then a private citizen.
Rest here:
http://www.senate.gov/artandhistory/history/common/briefing/Senate_Impeachment_Role.htm