Julian Zelizer is a professor of history and public affairs at Princeton University and author, with Kevin Kruse, of the new book "Fault Lines: A History of the United States Since 1974." Follow him on Twitter at @julianzelizer. The opinions expressed in this commentary are his own. View more opinion articles on CNN.

(CNN) President Donald Trump is doing his best to obstruct the investigations into him, his campaign and his administration being carried out by the Democrats who control the House of Representatives. Trump is warning that he will refuse to allow key documents -- such as his tax returns -- to be released to the relevant congressional committees and that he won't allow administration officials to testify. "We're fighting all the subpoenas," the President told reporters as he got on a plane for Atlanta. He even said, "If the partisan Dems ever tried to Impeach, I would first head to the US Supreme Court."

Julian Zelizer

House Speaker Nancy Pelosi is mad as hell. Her spokeswoman released a statement saying: "President Trump and his Administration are engaged in unprecedented stonewalling and once again using the legal system to conceal every area of his life as well as his wrongdoing and improprieties from the American people."

We are getting deeper and deeper into a constitutional crisis.

According to the principle of executive privilege, presidents have the right to keep information away from Congress or the courts under certain conditions. A handful have used a number of arguments to justify executive privilege, including the interests of national security or the need to allow staffers to have conversations free of the fear they will be on the front pages.

Trump would not be the first president to utilize this privilege, if that is the argument he goes with. Presidents since George Washington have invoked the right to keep documents and advisers away from Congress under certain circumstances. The term was introduced by President Dwight Eisenhower when he refused to share information or advisers with the Army-McCarthy hearings . "Any man who testifies as to the advice he gave me won't be working for me that night," Eisenhower warned.

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