John McCain and his running mate are both indeed political outsiders by character. Their record of going against the Republican establishment—McCain in Washington, DC and Sarah Palin in Alaska—is undeniable and the designation of "maverick" has been succesfully affixed by sheer brazen repetition at this week's party convention. The Obama campaign's response—even after Palin's unusual performance last night in St. Paul—has been merely to repeat that the supposedly independent-minded hero at the top of the ticket has in fact voted with George Bush 90% of the time. Wrong answer. McCain's campaign has admitted to the candidate's greatest vulnerability: it's precisely because he's such a maverick that voters shouldn't trust him with power. The Democrats should accept McCain as a maverick—a dangerous maverick—and turn that quality against him.

Before he appeared on the national stage, Karl Rove once explained the key to effective negative campaigning. "Look, I don't attack people on their weaknesses," he once told reporters in Texas during a campaign. "That usually doesn't get the job done. Voters already perceive weaknesses. You've got to go after the other guy's strengths. That's how you win."

That was the tactic employed in 2004 by conservative groups challenging John Kerry's national security credentials; they undermined the Democratic candidate's credibility as a war hero. And it's the very same tactic that McCain campaign manager Steve Schmidt has adopted to deal with a much more formidable opponent, Barack Obama. The first round of negative ads were straight out of Karl Rove's playbook. The Democratic contender was portrayed was not so much a charismatic leader as a celebrity; driven not so much by destiny as hubris.

If one were to attack John McCain at his point of greatest strength, what would that be? Not his affiliation with the Republicans. In Rove's words, voters already know that (to the extent that it's true.) Even if the McCain campaign removes all mention of George Bush from the speeches and Republican signage from the halls, the party cannot escape the last eight years. John McCain strongest suit appears to be his maverick nature; he is a man who puts principle before party.

That doesn't sound too bad on the surface. Trust-busting Theodore Roosevelt was an eccentric but successful president. In the UK, Winston Churchill was a voice in the wilderness until the country turned to him in desperation in 1940. (It's no surprise that these are two of McCain's political heroes.) The maverick is a so attractive an American archetype that they made a TV show around the a character with that name, an adventurous gambler played by James Garner.

John McCain is indeed adventurous; his selection of an untested running mate whom he had met only once attests to his willingness to take a gamble. Even at the cost of popularity in his party, McCain has often been a politician of stern principle. These are engaging qualities; and ones shared to some degree by Sarah Palin, the vice-presidential nominee. The pair were presented this week by some supporters as "maverick squared."

But these are the attributes not of an American president but of a defiant prisoner in a Hanoi prison camp; an unbowed dissident in the Soviet Union; or head of state in one of those countries with a presidency sufficiently powerless that it can be given as a lifetime achievement award to the keeper of a nation's conscience.

By contrast, the American presidency is an executive role. Decisions require deliberation; principle must be put to one side in the interest of a messy compromise; pride must be swallowed. My personal test is a hypothetical reenactment of the Cuban Missile Crisis. If McCain were president, could he really ignore the more belligerent rantings of America's enemies? Would he, like Jack Kennedy, have made the face-saving concession that helped the Soviet Union withdraw missiles from Cuba? If the phone rings at 3am in the White House, it's McCain the proud martyr I worry about rather than careful Barack Obama.

Obama may indeed have less experience in politics than John McCain. It is slightly unsettling that a man so ambitious never filled out his resumé with a management job. But he is at least deliberate in his thinking and decisionmaking; one can imagine him as the boss of a company; he has the temperament of a chief executive. John McCain, the maverick, doesn't.