The Speech and Debate Clause

The current focus of the effort's of a Georgia district attorney to subpoena U.S. Senator Lindsey Graham.

The clause can be found in Article One, Section Six of the U.S. Constitution. It reads as follows: 

They [U.S. 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.   

It is intended to shield the legislature from the judiciary, which means that it is one of the many checks and balances within the U.S. governing system.

Background: 

- Constitution Annotated.

- Legal Information Institute.

- Wikipedia.

- Congressional Research Center.