https://definitions.uslegal.com/c/congressional-immunity/
Congressional immunity refers to a special immunity that is granted to members of Congress. The following are the exemptions granted to the members of Congress under congressional immunity :
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A Congress member is exempted from arrest while attending a session of the body to which the member belongs, excluding an arrest for treason, breach of the peace, or a felony, or;
2.A Congress member is exempted from arrest or interrogation for any speech or debate entered into during a legislative session.
In In re Grand Jury, 821 F.2d 946 (3d Cir. Pa. 1987), it was held that congressional immunity is an evidentiary privilege that is applied broadly to evidence or testimony about all acts that occur in the regular course of the legislative process. It therefore applies to activity taken in the course of legislative fact-finding.