It is within Democrats’ power to hold Trump officials in contempt and detain them as they testify before Congress until the whistleblower materials they’ve subpoenaed are delivered. They can pledge to freeze the director of national intelligence and attorney general office budgets until their officials cooperate fully with House investigations. They can refer Giuliani and Trump to the Justice Department for criminal investigation and haul FBI Director Christopher Wray up to the Hill under the expectation that he’d disclose whether he’d opened such an investigation. They could subpoena Giuliani himself, and hold him in contempt if he refuses to appear or answer questions about his conversations with the Ukrainians. These and other steps might pry the materials loose, or corner Trump into pardoning Giuliani, which would itself be an impeachable offense.
From Trump's Ukraine Scandal Is Our Last Chance to Get It Right by Brian Beutler