Expulsion and impeachment are both political, not criminal proceedings. I'm pretty sure you could expel a congressman for no reason at all if you have the votes, but no one would do that because of the precedent it would set.
I'm too lazy to look them up at the moment, but I know there are expulsion precedents from the Civil War era in regards to the Southerners' seats.
WyldeWolf1 The Man of 1,008 holds, making him 3 holds better than Chris Jericho!
from THEONION.COM: WASHINGTON, DC—Amid growing anti-war protests and polls indicating eroding public support for an invasion of Iraq, President Bush is offering U.S. taxpayers a rebate in the amount of $300 if we go to war.