Can congressional leaders “expel” elected members of Congress?

Question

According to a Washington Post article about Alabama Senate candidate Roy Moore, in the spotlight for an apparent history of sexual relations with teenagers, congressmen are trying to...

pressure GOP nominee Roy Moore to withdraw from the Alabama Senate race amid allegations of sexual misconduct, declaring him “unfit to serve” and threatening to expel him from Congress if he were elected.

That last part is the interesting one. Do members of Congress have the power to "expel" a democratically (if controversially) elected senator from the United States Congress?

Answer 1

The leaders can't do it unilaterally, but the members collectively can expel other members. It requires a two-thirds vote of the Senate.

US Constitution, Article I, Section 5:

Each House may determine the Rules of its Proceedings, punish its Members for disorderly Behaviour, and, with the Concurrence of two thirds, expel a Member.

Normally this would be preceded by a committee investigation, which might issue a recommendation as to whether the member should be expelled.

To date, fifteen US Senators have been expelled via this process: see https://www.senate.gov/artandhistory/history/common/briefing/Expulsion_Censure.htm. Fourteen of them were in 1861 for supporting the Confederacy, and the other was in 1797 for "Anti-Spanish conspiracy and treason". There were several more cases in which the Senate considered expulsion but ultimately voted not to, and others in which the Senator in question resigned under threat of expulsion.