What’s the threshold for expulsion from the Senate?

Article I, Section 5, of the United States Constitution provides that “Each House [of Congress] may determine the Rules of its proceedings, punish its members for disorderly behavior, and, with the concurrence of two-thirds, expel a member.”

Since 1789, the Senate has expelled only fifteen of its entire membership. Of that number, fourteen were charged with support of the Confederacy during the Civil War. In several other cases, the Senate considered expulsion proceedings but either found the member not guilty or failed to act before the member left office. In those cases, corruption was the primary cause of complaint.

Senators take an oath of office. Violating that oath is presumably grounds for expulsion but what about behavior prior to taking the oath?  Should prior behavior be fair game?

If prior behavior is fair game then should only criminal convictions be considered or is allowable to consider socially-inappropriate but non-criminal behavior also?

If prior behavior is to be considered is the threshold for expulsion that the behavior in question violated the “will support and defend the Constitution of the United States against all enemies” portion of the oath or is there a different standard?  If so then what is it?

Expulsion is extreme. What’s the threshold for demanding a Senator’s resignation? How is it different from expulsion and what’s the rationale for the particular differences?

Some history: Prior to an expulsion vote, Sen Williams resigned in 1982 for taking Abscam bribes while in office. Sen Packwood resigned in 1995 after allegations of sexual assaults committed while in office. Sen Ensign resigned in 2011 after disclosure of improprieties committed while in office.

Sen. Durenberger was censured in 1990 for financial improprieties while in office. He plead guilty to misuse of public funds in 1995. The most recent expulsions were in 1862 – as noted above, for supporting the Confederacy.

Please think hard about thresholds for expulsion and demanding a resignation. There’s no obligation to comply with precedent but be careful what you wish for. Whatever rules you come up with, be creative in imagining how others might choose to apply them.