The Speech or Debate Clause in Article I, Section 6 states that senators and representatives "shall not be questioned in any other Place" for "any Speech or Debate in either House." This gives members absolute criminal and civil immunity for legislative acts performed during official duties.
The Framers created this protection to prevent presidents or executive officials from arresting members on false pretenses to block votes or obstruct legislative actions they opposed. When courts determine members are acting within the "legitimate legislative sphere," the clause blocks all lawsuits or prosecution.
The Supreme Court narrowed this protection in United States v. Brewster (1972) and Hutchinson v. Proxmire (1979), ruling that criminal actions and political speech outside official duties don't receive immunity. The clause protects the legislative process—including drafting bills, committee work, floor debates, and voting—but doesn't shield lawmakers' personal conduct or communications with constituents outside Congress.