WASHINGTON -- The most useless speculation today in Washington is whom Chief of Staff Josh Bolten might choose to replace Karl Rove at the White House. He is genuinely irreplaceable. Nobody will attempt to combine the political and policy functions as Rove has done. Indeed, fellow Republicans question whether he should have attempted the feat himself.

Rove was a principal target of congressional Democrats even before February 2005, when he became deputy chief of staff in addition to senior adviser to President Bush. But the combination of the duties intensified the assault on him. Prominent Republicans of late have privately expressed the desire that he leave the government in hopes that might diminish the intensity of the Democratic assault.

While Rove decamping back to Texas is unlikely to defang the opposition, the mere fact that it is mentioned as a possibility reveals the ambiguity of his legacy. Rove is one of the canniest and most successful managers in American political history. Yet he is viewed within his own party's ranks, especially on Capitol Hill, as part of the problem afflicting the Grand Old Party.

Rove is unique, a rare political mechanic with a comprehensive knowledge of American political history. As an obscure young campaign consultant in Austin, Texas, 20 years ago, he embraced George W. Bush -- who had failed in both politics and business -- and gave him a plan to guide him into the White House.

But that victory in the 2000 election was so narrow -- a margin of less than 1,000 votes in Florida and one Supreme Court justice -- that it brought with it Democratic rage at Bush as an "illegitimate" president. In reaction, Rove went to work to build a stronger Republican base, reaching out to Bush administration officials for party building. That is nothing new in American politics, but has seemed more blatant the last six and one-half years.

The combination of party and policy was epitomized by the distribution in the White House of Republican National Committee e-mail accounts, with presidential aides given party BlackBerrys. This lethal melding was confirmed after the 2004 election victory, when Rove as deputy chief of staff took on policy as well as political duties.

He was at that point heralded in GOP ranks as a master politician, designing a ringing Republican victory in the 2002 midterm elections sandwiched by his guiding a flawed candidate to two presidential victories. But gratitude in politics is not forever. Republican congressional cheers turned to jeers after the 2006 losses.

Rove has not helped his popularity on Capitol Hill in his talks to new congressional candidates for 2008 that blame the 2006 elections on profligate spending and numerous scandals by the Republican-controlled Congress. To many Republicans in Congress, the Democratic victory can be traced to the Iraq war and a decision by Bush and Rove to "nationalize" the midterm elections.

Rove always had been a happy warrior, self-confident in building a broad-based Republican majority. But his joy of politics was diminished by Special Counsel Patrick Fitzgerald's investigation of him in the CIA leak case. Although Fitzgerald knew from the start that not Rove but the politically nondescript Deputy Secretary of State Richard Armitage was my primary source in identifying Valerie Plame as a CIA employee, the prosecutor came close to indicting Rove for perjury or obstruction of justice. Rove rivaled Bush as a hate-figure for left-wing politics.

Joseph Wilson did not know the identity of my actual source when he talked about "frog-marching" Rove into jail, setting a mindless pattern soon followed by bloggers and politicians alike. A talkative juror, after convicting Scooter Libby for perjury and obstruction of justice, expressed sorrow that it was not Karl Rove.

The desire to get Rove has outlived the Valerie Plame case, with Democratic lawmakers trying to make him the target in the fired U.S. attorneys case. Since there will be no impeachment proceedings against the president, Rove has been the best available surrogate.

No wonder that a leading Republican has been asking around whether ferocious Democratic partisans in Congress might ease up if Rove were no longer there to kick around. That provides melancholy exit music for one of the most effective, most powerful of all presidential aides.