The Bush White House’s latest assertion of how this administration is totally above the law, and answerable to nobody (except possibly the captive in-house Supreme Court), has already led to renewed calls for impeachment (see this, for example). It’s safe to predict that more and more people are going to start seriously calling for impeachment.
Yes, the Democrats should launch impeachment proceedings immediately. We may as well force them to spell out right away how and why the President can no longer be impeached without his assent. It’ll save the time and energy they are no doubt expending right now “discussing when and how to disclose” this stance.