The Supreme Court has long since ruled that states have the right to restrict the voting rights of serious criminals, which also restricts them from running for office. (That is because, to be a candidate in most states you must be qualified to be a voter.)
There is no "unfairness" in this, since as Robert Blake used to say, before he was charged with murder himself, "If you can't do the time, don't do the crime." Losing your voting rights is just one more consequence of committing a serious crime -- in some states, but not in others.
Congressman Billybob
Click for "Til Death Do Us Part."
In the word's literal sense, former congressmen qualify as EX-CONs....