This question, however, is far from settled. Because inter-branch disputes are generally resolved through negotiations to avoid a constitutional crisis, there is little case law defining the contours of the privilege. So it’s unclear whether Trump would succeed in asserting executive privilege in a Senate impeachment trial. Presumably, if senators subpoenaed a witness, such as Bolton, and Trump invoked privilege, the Senate could hear a motion to compel the testimony. Chief Justice John G. Roberts Jr., as the presiding officer, could issue an order to make the witness appear. It is unclear whether Trump could file a separate lawsuit in a federal court to quash the subpoena. It seems possible, if not likely, that a court would dismiss such a lawsuit as a non-justiciable political question that must be decided by the Senate, to which the Constitution gives with the “sole” power to try cases of impeachment. At the very least, Trump could delay proceedings in the Senate by filing a lawsuit challenging the subpoena and appealing it to the Supreme Court (where Roberts would find himself in an odd position, having already been involved in the case in the Senate trial).
But now, the executive privilege argument is no longer available. Trump’s tweets directly denying the substance of Bolton’s reported allegations waive any privilege that might have protected them from public disclosure. Privilege is meant to keep a president’s secrets confidential. If the president reveals those secrets or publicly discusses the conversations himself, there is no longer any need to protect them from disclosure.