Sounds nice, but will it get votes?

Republicans have nominated Arizona Senator John McCain as their presidential candidate, describing him as a man who "brings a lifetime of experience to the campaign," who exhibits "personal courage and heroism" and who "has shown his independence of right-wing orthodoxy on some very important issues". And that's just what Hillary Clinton, Joe Biden and Bill Clinton have to say about him.

In the occasionally entertaining and often bizarre menagerie that is today's Republican Party, McCain is that rarest of animals: someone whom plenty of Democrats like. This isn't an accident.

Over the years, McCain has cultivated a reputation for bipartisanship, for confronting interests in both parties, for putting "country first". That type of selflessness pays political dividends. In a year when partisanship is a liability, McCain can plausibly claim that he is the true "post-partisan" in the race for the White House.

He has crossed more party lines, angered more ideologues and won over more converts than Senator Barack Obama. And it may cost him the presidency.

This is less of a paradox than it may seem. Obama has spent only a few years in Congress, where he has quickly fallen into lockstep with his party's left wing. The Republicans with whom he has joined hands are few. McCain, however, is a creature of the Senate.

He understands that bipartisan cooperation and compromise are the way to pass legislation there. But what works for a senator doesn't necessarily help a presidential candidate. Lord Palmerston may as well have been talking about US politics when he said that there are no permanent allies, just permanent interests. And the permanent interest of every politician is to gain more power. Allies are ancillary.

Consider the line-up of speakers at the Democratic National Convention in Denver recently, which so often resembled a who's who of McCain's close friends and collaborators. There was Ted Kennedy, with whom McCain has worked to reform America's dysfunctional immigration policy; Tom Daschle, who worked with McCain in the 1990s to defang the tobacco industry; John Kerry, with whom McCain voted against the 2001 and 2003 Bush tax cuts.

Don't forget the Clintons, either. Hillary is McCain's erstwhile drinking buddy. And McCain supported her husband's 1999 intervention in Kosovo, trying hard to rally Republicans around an assertive foreign policy.

Litany of attacks

What did all this bipartisanship get McCain? A litany of attacks. In Denver, Democrats charged that McCain's presidency would be another four years of George W. Bush's. McCain, the Democrats said, is not the "change we need". In fact, he will be "more of the same" and will lead a "government where the privileged few come first and everyone else comes last."

Aisle-crossing may have helped McCain pass legislation. But it has neither inoculated him against Democratic criticism nor eased the concerns of ideologues in his own party. The research remains inconclusive, but John McCain is probably the single leading cause of heartburn among conservative activists.

Right-wingers passionately defend McCain's support for the surge strategy in Iraq, true. And his deviations from party orthodoxy lend credence to his postpartisan reputation. But that's the problem. Those deviations also nearly cost him the GOP nomination. And they dampen party regulars' enthusiasm for his candidacy.

McCain's ideological flexibility has broadened his appeal among independents and Democrats. But that flexibility is also a trap. To quell the incipient rebellion on his right during the Republican primaries, McCain shifted in a conservative direction on a variety of issues. He made amends with the Reverend Jerry Falwell, whom he had once called an "agent of intolerance".

He stopped talking about regularising the status of illegal immigrants already in the United States and called to renew and extend the Bush tax cuts. He thereby opened up a space for Democrats to accuse the "maverick" of caving in to the right wing.

It's a space that McCain's opponents are happy to fill.

Post-partisanship makes for good headlines. It heightens the self-esteem of goo-goo sophisticates who want to be above disagreement. But the truth is that you can't have democratic politics without disagreement.

And that is the postpartisan's dilemma. Partisans will happily pull the lever for one of their own. But how many will turn out for a maverick?

Matthew Continetti, an associate editor at the Weekly Standard, is the author of The K Street Gang: The Rise and Fall of the Republican Machine.