the ability of congress to override a presidential veto
Veto If the Congress overrides the veto by a two-thirds vote in each house, it becomes law without the President's signature. Otherwise, the bill fails to become law unless it is presented to the President again and the President chooses to sign it.[12] Historically, the Congress overrides the Presidential veto 7% of the time.[13]
Contempt of Congress Congressional rules empower all its standing committees with the authority to compel witnesses to produce testimony and documents for subjects under its jurisdiction. Committee rules may provide for the full committee to issue a subpoena, or permit subcommittees or the chairman (acting alone or with the ranking member) to issue subpoenas.