"Article I, Section 6, Clause 1, of the U.S. Constitution states in part,
for any Speech or Debate in either House, [senators and representatives] shall not be questioned in any other place.The purpose of the clause is to prevent the arrest and prosecution of unpopular legislators based on their political views."
This only applies to when they are speaking from the floor which is the only time that Reid is so adamant that Romney has not paid taxes! He doesn't say (while on the floor) that there is a rumor, he flat out say, Romney has not paid taxes!
So this clause protects their lies and innuendo's.