The United States v. Helstoski case in 1978 revolved around the interpretation of the Speech or Debate Clause, which provides immunity to members of Congress for their legislative activities. Former Congressman Henry Helstoski was indicted on charges related to accepting bribes in exchange for introducing private immigration bills. The government used evidence from his past legislative acts as part of its prosecution strategy. However, Helstoski argued that this violated the Speech or Debate Clause and moved to dismiss the indictment. The Supreme Court ruled in favor of Helstoski, stating that any evidence relating to past legislative acts is not admissible under this clause and cannot be used against a member of Congress during a criminal trial. This decision reinforced Congressional immunity under the Constitution's Speech or Debate Clause by protecting legislators from being prosecuted based on their legislative actions.

In the dissenting opinion for United States v. Helstoski, Justice Brennan argued that the Speech or Debate Clause should not be interpreted to bar evidence of legislative acts in a prosecution against a congressman for bribery. He contended that such an interpretation would provide members of Congress with absolute immunity from criminal liability and thus undermine the fundamental principle that no one is above the law. Instead, he suggested that while this clause protects legislators from being questioned about their legislative activities, it does not shield them from prosecution based on independently obtained evidence of illegal conduct related to those activities. Therefore, according to Brennan's view, any incriminating documents found during investigations could be used as evidence in court without violating constitutional protections.