You can tell something about a man from the enemies he has. And John McCain has made many in his years in public life – most of them passionate Republicans. Here’s Rush Limbaugh, high priest of the Republican base, on the subject of the Republican presidential candidate: “Senator McCain’s domestic record is not conservative, and we’re being lectured by the media – some who are hostile to conservatism, some who wear the conservative label – to be quiet, to not be too hard on him, or whatever. Unlike McCain, [Bob] Dole didn’t lie all the time while claiming to engage in straight talk.”

Lie? Yes, the main conservative criticism of McCain is that he’s a liar, a deceit artist, a closet left-liberal just itching to destroy conservatism from within.

To most people not enveloped in Washington political insiderism, this is a little counter-intuitive, to say the least. McCain looks pretty conservative to the public. After all, he is a scourge of government spending; he has always been against the legality of almost all abortions; he supported his own state’s constitutional amendment to ban gay couples from getting married; he was a passionate supporter of the Iraq war, the most prominent advocate of the recent troop surge in Baghdad, and was a loyal Bushite in 2004 even after being beaten brutally by the Bush machine in the South Carolina primary in 2000.

A liar? This is a man who is almost pathologically indiscreet, a press-gabber of monumental proportions, a character as candid in public as he is in private. He is also a bona-fide war hero who was imprisoned by the Viet-cong and a politician who has paid more than his dues over the years as a party stalwart.

So what on earth is the Republican base’s problem? McCain’s cranky 94-year-old mother summed up the situation last week when asked: “How much support do you think he has among the base of the Republican party?” She replied: “I don’t think he has any.”

Ann Coulter, the ferociously right-wing columnist who has also called McCain a liar, explains her charge thus: “I might lie constantly, too, if I were seeking the Republican presidential nomination after enthusiastically promoting amnesty for illegal aliens, social security credit for illegal aliens, criminal trials for terrorists, stem-cell research on human embryos, crackpot global warming legislation and free speech-crushing campaign finance laws. I might lie, too, if I had opposed the Bush tax cuts, a marriage amendment to the constitution, waterboarding terrorists and drilling in Alaska.”

This is worth unpacking because it reveals, like a photographic negative, why McCain really is the best hope for the Republicans in moving past the Bush-Cheney years. On all the issues that Coulter mentions, McCain has been a little bit of a dissident but in all the right ways.

On immigration, McCain has backed a bipartisan approach that both ramps up the border fence with Mexico and tightens restrictions on employers of illegal aliens, and yet offers a path to citizenship for the 12m illegal immigrants in the United States. To the far right this is amnesty. To most people it’s sanity. Rounding up and deporting millions and crippling large sections of the agricultural, service and construction industries is the far right’s preferred policy, but it is practically a nonstarter.

On climate change, McCain actually concedes that it exists, is primarily man-made and requires some kind of response. This is supposed to be a liability? What planet are the Republicans on?

On marriage rights, McCain’s sole concession to the centre is that he does not believe that the federal constitution needs to be amended to prevent the states’ experimentation with allowing same-sex weddings in some jurisdictions.

On waterboarding suspects as part of interrogation in US prisons, McCain sticks to the Geneva conventions and America’s duty to uphold them with respect to terror suspects. Again: in what universe is this anathema?

To my mind these sensible, traditional and pragmatic conservative positions are definitely up for debate. But the idea that they render a Republican of McCain’s stature unthinkable as president is just unhinged.

That, sadly, is what the Republican party base has become these past few years. With each lurch to the big spending, big government, authori-tarian right, the remaining moderates or even yesterday’s Reagan conservatives have been tarred as traitors or insufficiently sound or impermissibly wimpy. And this has continued even though the Republicans’ popularity has plummeted, even though none of the anointed base Republican candidates has shown he can win a majority, even though the Republican coalition, under pressure from the nasty purists, has finally, incontrovertibly, cracked into many little pieces.

In that sense, perhaps the real change candidate in this election is not the charismatic, youthful, biracial voice of reason in Barack Obama. Perhaps it is the crustiest candidate of the old generation, the white-haired, stiff-armed, beaming, 71-year-old veteran of campaigns past, threatening to purge Republicanism of the ugliness and divisiveness that have marred it in recent years.

The polls attest that he is easily the best positioned of the Republicans to win a majority in November, especially if Hillary and Bill Clinton are his opponents. And if he were to win, he would throw a mighty wrench into the direction that conservatism has taken these past few years.

His main opponent now is Mitt Romney, a Mormon who invented his candidacy from scratch by examining and echoing the most extreme base views. Romney was once a pro-gay, pro-choice pragmatist. He is now a man who wants to ban all abortions and same-sex unions by constitutional amendment and says that atheists are not fully part of the American experiment.

Few people think that Romney is anything but an opportunist; he is widely loathed by his fellow candidates and he has already sunk $100m into a campaign that has failed to take off. In potential match-ups with both Clinton and Obama, he lags by double digits. And yet the Republican purists would prefer a fake loser who could echo their talking points than a potential majority president who is open to compromise and change.

So far, ordinary Republicans and independents have kept the McCain candidacy alive. What we will find out in the next couple of weeks is whether the bitter purists can still kill it. Florida is the key test: if McCain can win there, he will be close to unstop-pable. But he has not broken 30% there, even with the momentum of two clear primary wins behind him.

It is in so many ways a last hurrah. Not just for McCain, but also for a renewed conservatism that can breathe and think and appeal again. Sometimes it takes the oldest codger to prompt the hardest growth. How barren is the topsoil? We will soon find out.

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