It is all too easy to underestimate the significance of President Donald Trump's outright defiance of Congress' request that senior aides testify. His declaration that this White House will resist all congressional subpoenas, across the board, is an unprecedented challenge to the House's exercise of its constitutional oversight powers.
To be sure, presidents and Congress joust often over requests for senior aide testimony or White House documents. The public might suspect that such disputes are invariably partisan, with Democratic presidents resisting Republican requests for information and vice versa.
In fact, the executive and the legislature have, at least until now, fought these battles within a shared, if tense, understanding of the constitutional precedents: Congress has extensive authority to probe an administration's activities and performance, while presidents retain appropriate latitude to solicit, and keep private, advice from senior staff.