If the gambit stands, it suggests a President can simply ignore the tools designed for Congress to check his power and examine his actions and he will face no significant sanction. Once this threshold is crossed there may be no going back for future administrations.

“This is not a normal partisan event, because the President is invoking executive privilege in an extremely broad way,” said Corey Brettschneider, professor of political science at Brown University.

“It is hard to think of an example like that, that is as fundamental to the American Constitution, not only that Congress makes the laws and the president exercises them, but part of the legislative function is to exercise oversight.

“Without the subpoena power to compel oversight it is hard to know how they can even do the basics of their jobs,” said Brettschneider, author of “The Oath and the Office: A Guide to the Constitution for Future Presidents.”