Never mind Sarah Palin. John McCain has another running mate: Gen. David H. Petraeus.

Mr. McCain didn't mention Ms. Palin once by name during his first debate with Barack Obama. He brought up Gen. Petraeus seven times. He's been invoking the Iraq commander in speeches around the country -- in Ohio and Pennsylvania, Virginia and Florida. He's even been carrying around a biography of the general. And he has schooled Ms. Palin, who in Thursday's debate called Gen. Petraeus "a great American hero."

For Mr. McCain, Gen. Petraeus is the icon of the turnaround in Iraq brought about by the surge of troops that Mr. McCain vociferously supported. "Thanks to this great general, David Petraeus, and the troops who serve under him, they have succeeded, and we are winning in Iraq," he said in the Oxford, Miss., debate.

Gen. Petraeus is also Mr. McCain's battering ram against Mr. Obama. "Obama didn't go to Iraq for 900 days, and never asked for a meeting with Gen. Petraeus," Mr. McCain charges. Gen. Petraeus, he says, thinks the Democrat's plan to withdraw U.S. combat forces is "dangerous." Gen. Petraeus says that progress in Iraq is fragile, so "we can't do what Sen. Obama wants to do." And the general says Iraq is the central front in the war on terrorism -- not Afghanistan, as Mr. Obama would have it.

When was the last time a serving U.S. military officer was made to play such a large role in a presidential campaign, even if involuntarily? There may be no modern precedent. Colin Powell had retired from the Army by the time presidential candidates came courting him. Robert Taft tried to use Douglas MacArthur in the 1952 campaign but only after Harry Truman had fired the general.

Gen. Petraeus is preparing to take over his biggest assignment yet -- chief of U.S. Central Command, with responsibility for the entire Near East, including Iraq, Afghanistan, Pakistan, Iran, Syria and Lebanon. He will be at the center of the next administration's decisions about how to manage two wars, not to mention the gravest threats of terrorism and nuclear proliferation. Yet Gen. Petraeus is being employed by one candidate to prove his worth, and discredit his opponent, in a contest to determine which of them will become the general's next commander in chief.

Not all of this can be chalked up to crude seeking of political advantage. Mr. McCain undoubtedly feels he has a connection to Gen. Petraeus. They both pressed for a strategy of sending five additional brigades to Iraq at a time when most of Washington -- and most of the Pentagon -- opposed it. Mr. McCain made a high-profile visit to Baghdad early in the surge to endorse what was then its uncertain results -- and for his trouble was ridiculed for walking through a Baghdad market with overwhelming security backup.

It's true, too, that Mr. Obama's relationship with Gen. Petraeus is tenuous. At the debate, Mr. Obama praised the general, saying he "has done a brilliant job" in Iraq. But during his own visit to Baghdad in July, Mr. Obama was candid enough to concede that he and Gen. Petraeus did not agree about troop withdrawals. He sought to finesse the difference, drawing a distinction between what a commander in one war theater might recommend and the president's responsibility to make a broader judgment: "I'm factoring in their advice but placing it in this broader strategic framework," he said.

In fact, if Mr. Obama is elected, his management of Gen. Petraeus may be the most important early test of his administration. Though progress in Iraq has narrowed their differences, Gen. Petraeus and his new deputy in Baghdad, Gen. Ray Odierno, will almost certainly tell the new president that withdrawing U.S. combat troops from Iraq at the rate of a brigade a month next year, as Mr. Obama has proposed, would be too risky.

Would Mr. Obama listen to the most lauded war general since MacArthur or conspicuously overrule him? Gen. Petraeus, a skilled politician in his own way, would certainly seek to avoid confrontation. But if there is to be a consensus, Mr. Obama would need to retreat from his campaign rhetoric.

The irony is that Mr. McCain pushed for Gen. Petraeus' appointment to Central Command in part to constrain the next president. As Bob Woodward reports in his new book, Mr. McCain was swayed by retired Gen. Jack Keane, a backstage author of the surge, who argued that once Gen. Petraeus "took over the command, the U.S. strategy in the Middle East would be locked in, no matter who won the 2008 presidential race."

Mr. McCain's use of Gen. Petraeus in the campaign could very well undo that lock, if the general comes to be tainted as a Republican tool. Advisers to Mr. Obama tell me that his opinion of Gen. Petraeus hasn't been affected by Mr. McCain's campaign tactics. But the fact is that by employing the general so heavily in his rhetoric, Mr. McCain is doing a disservice to a commander whose skills the next president will sorely need -- especially if he is Mr. Obama.