Is someone guilty of treason and/or dishonorably discharged from the armed forces allowed to serve in the Senate?
Guilty of treason? No. But there has been no conviction.
Dishonorably discharged? Repugnant, but yes.
We've had far worse serving in Congress. For example, Alcee Hastings was a federal judge impeached and removed from the bench for apparently taking a bribe (believe there was an FBI video of it). Hastings then was elected to the U.S. House of Representatives as a Dimwit.
Congressman Barney Frank's boyfriend was running a male prostitution ring from his house. He is considered Dimwit leadership and was planning to take Kerry's Senate seat if Kerry had won the presidency.