The Brady Rule can force the House to disgorge impeachment documents
The Brady Rule can force the House to disgorge impeachment documents
The framers of the Constitution wished to protect the Executive, Legislative, and Judiciary branches of government from each other. To protect the Legislative branch from the Executive branch, they included the Speech and Debate Clause of the Constitution which immunizes Senators and Representatives from any criminal or civil claims that arise during the time they are carrying out their official duties. 

This Congressional protection has been to a much lesser and juridically divided extent extended to protecting Congressional documents from being subpoenaed by an outside legal authority.  On the other hand, in the 1963 case of Brady v. Maryland, 373 U.S. 83, 87(1963) the U.S. Supreme Court ruled that under the Due Process Clause of the 5th Amendment, as applied to the states under the 14thAmendment, the prosecution in a criminal trial must voluntarily disgorge and disclose all information exculpatory to the defendant. This was extended by the U.S Supreme Court to include information that would affect the credibility of a prosecution witness. Giglio v. United States, 405 U.S. 150 (1972).

Therefore, since the U.S. House of Representatives has embarked on “an official impeachment inquiry,” President Trump can file a lawsuit under the Brady 5th Amendment constitutional due process protections to force the House to disgorge all documents and communications relating to any claims of impeachment including all communications with the alleged whistle blower.

On September 24, 2019, against the backdrop of 6 American flags, based on reports of an “inappropriate” President Trump-Ukrainian President call, House Speaker Nancy Pelosi stated: “The actions of the Trump presidency revealed dishonorable facts of betrayal of his oath of office and betrayal of our national security and betrayal of the integrity of our elections. Therefore, today, I’m announcing the House of Representatives is moving forward with an official impeachment inquiry and directing our six committees to proceed with their investigation under that umbrellas of impeachment inquiry. . . . The President must be held accountable. . . . No one is above the law.”

The New York Times has reported that there were extensive contacts and drafts of the whistle blower’s complaint exchanged with Adam Schiff, Chairman of the House Intelligence Committee.  The fact that the whistle blower's pre-filing communications with the House members violated the whistle blower statute itself would produce a treasure trove of documents that could impeach the whistle blower. But even apart from the apparent foundational illegality of the creation of the original report, the exact documents exchanged between the Democrats and the whistle blower could likely better enable President Trump to take down not only his whistle blower accuser, but also expose any existing web of Democrat illegal actions which culminated in the whistle blower report itself - and beyond.

A threshold question is whether under an impeachment proceeding against the President of the United States the Due Process clause of the 5th Amendment is invoked.  That Amendment states: No person shall ... be deprived of life, liberty, or property, without due process of law.

Clearly, the office of the President of the United States is protected by that Amendment. The salary alone is $400,000 a year. Impeachment would be depriving President Trump of the office and his $400,000 salary.  Therefore, any House impeachment inquiry necessarily invokes the 5th Amendment, and all its attendant protections.  Any and all 5th Amendment due process protections afforded to an alleged federal criminal must be afforded to President Trump, including but not limited to the Brady Rule regarding exculpatory evidence disclosure.

Speaker Pelosi rightly stated the legal truism that “No man is above the law.”  However, the necessary corollary to that legal truism is, “No man is below the law either.”  President Trump is neither above the law, nor below the law. And as Justice William O. Douglas for the majority Brady Court enunciated, “The principle of Mooney v. Holohan is not punishment of society for misdeeds of a prosecutor but avoidance of an unfair trial to the accused. Society wins not only when the guilty are convicted but when criminal trials are fair; our system of the administration of justice suffers when any accused is treated unfairly." 

An inscription on the walls of the Department of Justice states the proposition candidly for the federal domain: "The United States wins its point whenever justice is done its citizens in the courts." A prosecution that withholds evidence on demand which, if made available, [373 U.S. 83, 88]   would tend to exculpate the accused or reduce the penalty, helps shape a trial that bears heavily on the defendant. That casts the prosecutor in the role of an architect of a proceeding that does not comport with standards of justice, even though, as in the present case, his action is not "the result of guile," to use the words of the Court of Appeals. 226 Md., at 427, 174 A. 2d, at 169.”

Should a President accused of crossing the impeachment threshold of “high crimes and misdemeanors” receive less 5th Amendment due process than any other American citizen accused of a federal crime?  Of course not.  And in Justice Douglas’ words in the Brady Court, as applied to the House prosecution if it fails to provide exculpatory information to President Trump, the Democrats who “withhold evidence on demand of [President Trump] which, if made available, would tend to exculpate [President Trump]. . .  cast the [Democrats] in the role of an architect of a proceeding that does not comport with standards of justice [of the United States of America.”
Furthermore, as for the conflict between the Speech and Debate Clause and the Due Process Brady requirements, the 5th Amendment Brady due process requirements trump the Speech and debate clause.  The  Speech and Debate Clause was meant to shield the Legislative branch from inappropriate prosecution and intimidation by the Executive branch. Here, the Legislative branch is not using using the the Speech and Debate clause as a shield, but as a sword with which to attack the Executive, while denying the Executive elemental 5th Amendment due process. The disgorged documentary and other evidence would not be used to criminally attack the legislative branch.  Therefore, the Due Process protections of a President under impeachment outweigh any claims of protection from discovery that the Democrats may claim under the Speech and Debate clause.