Congress and the White House appear to be headed for a constitutional showdown. The House of Representatives is poised to hold Joshua Bolten, the White House chief of staff, and Harriet Miers, a former White House counsel, in contempt for failing to comply with subpoenas in the United States attorneys scandal. If the Justice Department refuses to enforce the subpoenas, as seems likely, Congress will have to decide whether to do so. Washington lawyers are dusting off an old but apparently sturdy doctrine called “inherent contempt” that gives Congress the power to bring the recalcitrant witnesses in  by force, if necessary.

What we know that Congress has learned in its investigation of the purge of nine top federal prosecutors is disturbing. Cases appear to have been brought against Democrats and blocked against Republicans to help Republicans win elections. The stakes have grown steadily: it now seems that innocent people, like Georgia Thompson, a Wisconsin civil servant, may have been jailed for political reasons. Congress has a duty to find out what happened.

Mr. Bolten and Ms. Miers may have important evidence. When Congress subpoenaed them, however, both claimed executive privilege in ways that go far beyond what the law allows. Ms. Miers should, at the very least, have appeared and invoked the privilege in response to specific questions. Instead, she refused to appear at all. Mr. Bolten, who was asked to produce documents, should have said specifically which ones he believed to be privileged. Instead, he rejected Congress’s right to ask for the documents.

As a result, the House Judiciary Committee voted in the summer to hold Mr. Bolten and Ms. Miers in contempt. If the full House does, too  or if the Senate, which is also considering contempt, does so  then the United States attorney in Washington, D.C., will be responsible for taking Mr. Bolten’s and Ms. Miers’s cases to a grand jury. The problem is that the White House argues that the contempt of Congress law does not apply to presidential subordinates who claim executive privilege. At his confirmation hearings, Attorney General Michael Mukasey sounded as if he might agree with this intransigent position.