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, Guantanamo, or anything on which he can muster enough support. But at least until now, the U.S. political system has avoided the spectacle of a new Administration prosecuting its predecessor for policy disagreements. This is what happens in Argentina, Malaysia or Peru, countries where the law is treated merely as an extension of political power.

If this analogy seems excessive, consider how Mr. Obama has framed the issue. He has absolved CIA operatives of any legal jeopardy, no doubt because his intelligence advisers told him how damaging that would be to CIA morale when Mr. Obama needs the agency to protect the country. But he has pointedly invited investigations against Republican legal advisers who offered their best advice at the request of CIA officials.

"Your intelligence indicates that there is currently a level of 'chatter' equal to that which preceded the September 11 attacks," wrote Assistant Attorney General Jay Bybee, in his August 1, 2002 memo. "In light of the information you believe [detainee Abu] Zubaydah has and the high level of threat you believe now exists, you wish to move the interrogations into what you have described as an 'increased pressure phase.'"

So the CIA requests a legal review at a moment of heightened danger, the Justice Department obliges with an exceedingly detailed analysis of the law and interrogation practices -- and, seven years later, Mr. Obama says only the legal advisers who are no longer in government should be investigated. The political convenience of this distinction for Mr. Obama betrays its basic injustice. And by the way, everyone agrees that senior officials, including President Bush, approved these interrogations. Is this President going to put his predecessor in the dock too?