As Mr. Lieberman sees it, this is a fight for the soul of the Democratic Party — his moderate fair-mindedness against a partisan radicalism that alienates most Americans. “What kind of Democratic Party are we going to have?” he asked in an interview with New York magazine. “You’ve got to agree 100 percent, or you’re not a good Democrat?”

That’s far from the issue. Mr. Lieberman is not just a senator who works well with members of the other party. And there is a reason that while other Democrats supported the war, he has become the only target. In his effort to appear above the partisan fray, he has become one of the Bush administration’s most useful allies as the president tries to turn the war on terror into an excuse for radical changes in how this country operates.

Citing national security, Mr. Bush continually tries to undermine restraints on the executive branch: the system of checks and balances, international accords on the treatment of prisoners, the nation’s longtime principles of justice. His administration has depicted any questions or criticism of his policies as giving aid and comfort to the terrorists. And Mr. Lieberman has helped that effort. He once denounced Democrats who were “more focused on how President Bush took America into the war in Iraq” than on supporting the war’s progress.

At this moment, with a Republican president intent on drastically expanding his powers with the support of the Republican House and Senate, it is critical that the minority party serve as a responsible, but vigorous, watchdog. That does not require shrillness or absolutism. But this is no time for a man with Mr. Lieberman’s ability to command Republicans’ attention to become their enabler, and embrace a role as the president’s defender.

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On the Armed Services Committee, Mr. Lieberman has left it to Republicans like Lindsey Graham of South Carolina to investigate the administration’s actions. In 2004, Mr. Lieberman praised Defense Secretary Donald Rumsfeld for expressing regret about Abu Ghraib, then added: “I cannot help but say, however, that those who were responsible for killing 3,000 Americans on September 11th, 2001, never apologized.” To suggest even rhetorically that the American military could be held to the same standard of behavior as terrorists is outrageous, and a good example of how avidly the senator has adopted the Bush spin and helped the administration avoid accounting for Abu Ghraib.