Findlaw:

President George Bush has issued an instruction to his former White House counsel Harriet Miers to defy the House Judiciary Committee's subpoena. The Committee had sought to ask her about her role - and that of others in the White House - in firing a covey of United States Attorneys who were apparently not toeing the political line. Bush's instruction sent a very clear signal: As I wrote earlier, and as has been clear from the outset, he is looking for a fight.

By not responding to the subpoena, the President and Ms. Miers all but invited the House Judiciary Committee and, in turn, the House of Representatives to vote to deem her in contempt of Congress. It was a defiant, in-your-face insult to Congress. No president would do this unless he was quite confident of the outcome. Clearly, Bush's White House and Justice Department lawyers believe that the solidly conservative federal judiciary will grant them a favorable ruling, and that, in the process, they will greatly weaken congressional oversight powers, to the advantage of the White House.

In short, the Bush White House is not bluffing with this act of defiance. Rather, the White House truly wants to test, and attempt to expand, presidential power.