What congressmen say on the floor of the senate or house is fully protected speech. They could say lies, slander or even advocate anarchy or communism, literally anything they say is constitutionally, specifically protected as members of congress.
But, as far as I know, if they say it outside the walls of the chamber, they can be held liable for threats of violence or for libel. If there’s any constitutional scholars on this board they are free to correct me.