Reply #118: Because there's already a process in the Constitution for Members of Congress:

[NYC Liberal]

Expulsion.

The Senator you're talking about is kind of a tricky case. Basically, the House voted to impeach him, then the Senate voted to expel him (completely separate from the impeachment), then tried to hold a trial. But they dropped it because they decided they lacked jurisdiction. Though of course, it was never settled whether they lacked jurisdiction because he'd already been removed from office or because members of Congress can't be impeached.

Anyway, it's a moot point because the better path is expulsion. It's faster. Why would you bother with an impeachment in the House (majority vote) followed by a trial in the Senate (2/3rds vote), when you could just expel them with a 2/3rds vote in ONE house? (Unless of course, you were trying to expel a Representative and thought you'd have a better chance of getting the 2/3rds vote in the Senate as opposed to the House...)