In some countries, yes. It's called party defection, but I'm having a difficult time finding rules for U.S. House/Senate. Members can be expelled for "disruptive behavior", which can be interpreted in many ways. In our system, I believe that offending members are simply put to a vote where a 2/3 majority can expel them from Congress.
Regarding individual parties, the chairman of the party can conduct a vote to expel them from the party.
Is it really illegal?
In some countries, yes. It's called party defection, but I'm having a difficult time finding rules for U.S. House/Senate. Members can be expelled for "disruptive behavior", which can be interpreted in many ways. In our system, I believe that offending members are simply put to a vote where a 2/3 majority can expel them from Congress.
Regarding individual parties, the chairman of the party can conduct a vote to expel them from the party.