Eastland v. United States Servicemen's Fund
In Eastland v. United States Servicemen's Fund, 421 U.S. 491, 502-503 (1975), the U.S. Supreme Court held that the Speech or Debate Clause immunizes Congressmen from suits for either prospective relief or damages.
The Court held that the purpose of this immunity is to insure that the legislative function may be performed independently without fear of outside interference. Id. To preserve legislative independence, the Court has concluded that "legislators engaged 'in the sphere of legitimate legislative activity,' Tenney v. Brandhove, [341 U.S. 367, 376 (1951)], should be protected not only from the consequences of litigation's results but also from the burden of defending themselves." Dombrowski v. Eastland, 387 U.S. 82, 85 (1967).