According to the Constitution, a congressman and/or senator can say ANYTHING they want about ANYONE on the floor and can’t be held in any way liable in any body outside of that building.

The Senators and Representatives shall in all Cases, except Treason, Felony and Breach of the Peace, be privileged from Arrest during their Attendance at the Session of their respective Houses, and in going to and returning from the same; and for any Speech or Debate in either House, they shall not be questioned in any other Place.
— United States Constitution (Article I, Section 6, Clause 1)

While some academics consider this clause “practically obsolete”, it clearly gives cover for today’s dishonest partisan.

As congressmen and senators were originally paid $6 per day, protecting a politician’s personal business and/or assets was a necessity. In the age of professional politicians who now make around $725 per day (aside from their ballooned portfolios that vastly outperform their salaries), knowing their personal freedom and finances are shielded from the ramifications of slander or libel has brought out the worse in our elected.

Harry Reid put his foot in his mouth before regarding Rush Limbaugh and the gleefully repeated “phony soldiers” lie prompted by Media Matters. Reid was proven dishonest, didn’t have to apologize and never has because he knows there is no redress in any venue.

If a Member’s actions meet the “legislative process” test, his immunity is absolute; and that is so even if he has acted contrary to law.

In Eastland v. United States Servicemen’s Fund (1975), the Supreme Court acknowledged that the clause may shield Members from civil or criminal liability “even though their conduct, if performed in other than legislative contexts, would in itself be unconstitutional or otherwise contrary to criminal or civil statutes.”
— James L. Buckley, Senior Judge, U.S. Court of Appeals, District of Columbia Circuit

We’ll see how many more times federal politicians can slander private citizens, candidates or not, because this won’t be the last time.