From a New York Times editorial:
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While executive privilege is a common presidential tool, historians note that Mr. Trump’s usage is decidedly uncommon, if not unprecedented. Unlike his predecessors, who invoked privilege in specific cases, Mr. Trump has vowed that he will not cooperate with any congressional inquiry. He is effectively declaring lawmakers powerless over him. This, warn the experts, puts the nation in uncharted territory and threatens to erode its democratic foundations.
Presidents clash with Congress, at times fiercely. The founders wanted it that way. But in declaring war on congressional oversight, Mr. Trump is not looking to maintain a balance of powers. He is looking to blow up the scales.