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.

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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?

Mr. Obama seemed to understand the peril of such an exercise when he said, before his inauguration, that he wanted to "look forward" and beyond the antiterror debates of the Bush years. As recently as Sunday, Rahm Emanuel said no prosecutions were contemplated and now is not a time for "anger and retribution." Two days later the President disavowed his own chief of staff. Yet nothing had changed except that Mr. Obama's decision last week to release the interrogation memos unleashed a revenge lust on the political left that he refuses to resist.

Just as with the AIG bonuses, he is trying to co-opt his left-wing base by playing to it -- only to encourage it more. Within hours of Mr. Obama's Tuesday comments, Senator Carl Levin piled on with his own accusatory Intelligence Committee report. The demands for a "special counsel" at Justice and a Congressional show trial are louder than ever, and both Europe's left and the U.N. are signaling their desire to file their own charges against former U.S. officials.

Those officials won't be the only ones who suffer if all of this goes forward. Congress will face questions about what the Members knew and when, especially Nancy Pelosi when she was on the House Intelligence Committee in 2002. The Speaker now says she remembers hearing about waterboarding, though not that it would actually be used. Does anyone believe that? Porter Goss, her GOP counterpart at the time, says he knew exactly what he was hearing and that, if anything, Ms. Pelosi worried the CIA wasn't doing enough to stop another attack. By all means, put her under oath.

Mr. Obama may think he can soar above all of this, but he'll soon learn otherwise. The Beltway's political energy will focus more on the spectacle of revenge, and less on his agenda. The CIA will have its reputation smeared, and its agents second-guessing themselves. And if there is another terror attack against Americans, Mr. Obama will have set himself up for the argument that his campaign against the Bush policies is partly to blame.

Above all, the exercise will only embitter Republicans, including the moderates and national-security hawks Mr. Obama may need in the next four years. As patriotic officials who acted in good faith are indicted, smeared, impeached from judgeships or stripped of their academic tenure, the partisan anger and backlash will grow. And speaking of which, when will the GOP Members of Congress begin to denounce this partisan scapegoating? Senior Republicans like Mitch McConnell, Richard Lugar, John McCain, Orrin Hatch, Pat Roberts and Arlen Specter have hardly been profiles in courage.

Mr. Obama is more popular than his policies, due in part to his personal charm and his seeming goodwill. By indulging his party's desire to criminalize policy advice, he has unleashed furies that will haunt his Presidency.