Mark down the date. Tuesday, April 21, 2009, is the moment that any chance of a new era of bipartisan respect in Washington ended. By inviting the prosecution of Bush officials for their antiterror legal advice, President Obama has injected a poison into our politics that he and the country will live to regret.

Policy disputes, often bitter, are the stuff of democratic politics. Elections settle those battles, at least for a time, and Mr. Obama's victory in November has given him the right to change policies on interrogations,...