Commentary: Trump’s stonewalling of Congress is illegal. How far will he push it?

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 i