An amazing thing may be happening in Republican politics. Usually the economy and social issues decide who gets the party's presidential nomination (plus the GOP's ingrained habit of giving the prize to the senior man in line: most recently John McCain in 2008 and Mitt Romney the last time around). Not this time, however. The key issue may well be foreign policy – and the beneficiary could be someone who's anything but the obvious man in line.

I refer to Rand Paul, by training an ophthalmologist, by trade the junior senator for Kentucky, and by instinct a gadfly and provocateur who has laid bare the foreign policy rift in Republicans that has festered unresolved since George W Bush's disastrous adventure in Iraq.

The potential Republican field for 2016 is the most intriguing and open in years. There's New Jersey's governor Chris Christie, once everyone's favourite until the great bridge traffic jam scandal exposed him as politics' equivalent of Tony Soprano. There's Marco Rubio, Florida's young and gifted junior senator, and Paul Ryan, Mitt Romney's running mate from 2012. Rick Perry, the outgoing Texas governor who made a fool of himself four years ago, is on the comeback trail, minus his trademark cowboy boots but sporting black-rimmed spectacles aimed at giving him the gravitas he then so visibly lacked.

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Stir in Mike Huckabee who ran McCain close in 2008, and several talented state governors who could throw their hats into the ring, and there's a terrific race in the making. And that's without Jeb Bush, W's younger brother, ever Hamlet-like but a heavyweight by any standards. None has yet formally declared (bad form before November's mid-terms are out of the way, and the 2016 contest begins in earnest).

Many of them, however, are already trawling Iowa and New Hampshire, the first states up in the primary season. "I will be back a lot," Christie ominously declared during a visit to Iowa last week. But if there is a front runner at this absurdly early stage of proceedings, it's probably Rand Paul.

It's not so much that he holds wafer-thin leads in both states, according to recent, near-meaningless polls. Nor that he's the son of Ron Paul, who made quixotic bids for the White House in both 2008 and 2012, acquiring along the way a modest but ferociously dedicated following. Nor even that he's a born opportunist, supremely quick to spot and exploit an opening. The simple fact is that Paul Jnr has an appeal that extends into odd corners of his own party, and beyond.

Up to a point, he's a conventional low-tax, small-government Republican, much loved by the Tea Party. He's for guns and against gay marriage. What sets him apart, though, is the libertarianism he inherited from his father. Paul is pro-privacy and a sworn foe of the blanket NSA snooping revealed by Edward Snowden. He opposed renewal of the post 9/11 Patriot Act – an article of faith for most Republicans – on the grounds that it infringed individual liberties. He even did a real, 12-hour filibuster on the Senate floor in protest against the use of drones. And not least, he has a libertarian's distaste for foreign entanglements.

All of this allows him to venture into places where most Republicans don't. In March, he got a standing ovation after a speech to students at the liberal redoubt of UC-Berkeley. Now he's reportedly making tracks back to California to tap the tech moguls of Democrat-leaning Silicon Valley for money, and enlist young digital whizzes to help to hone a 2016 campaign.

Above all, though, the Paul siren song extends to foreign policy, on which his party is deeply divided. The differences in part reflect the eternal American clash between isolationism and interventionism, the former personified in GOP presidential races as recent as 1992 and 1996 by the old bruiser Pat Buchanan.

In fact, Paul is not a true isolationist, of the "stop the world, I want to get off" variety. He admits that America's armed forces have a role abroad, a role that includes permanent foreign military bases. But he's profoundly sceptical of the use of military force and of the US ability, trumpeted by Bush and the neocons, to create a democratic garden in the stony deserts of sectarianism and authoritarianism.

Right now, such views strike a deep domestic chord. The wars in Iraq and Afghanistan, the longest in American history, produced much misery and little tangible gain, either for the US or the two countries themselves. And now, wherever they look, Americans see only trouble: in Iraq and Afghanistan, both falling prey again to the radical Islam against which the US took up arms; in Syria and maybe soon in relations with Iran, as prospects of a genuine nuclear deal recede; and, of course, Ukraine and Gaza.

Nowhere does the US appear able to shape events. Obama's foreign policy approval ratings are accordingly terrible, but not because the President isn't doing what American voters want. His problem is that while the last thing they want is a new war, they simultaneously demand that the US appears strong. And right now, almost everyone would admit, it doesn't.

Paul is not even a formal candidate let alone a president, and doesn't have Obama's problem. Instead, he can carry the foreign policy battle to his rivals, pushing Republicans towards a reckoning on the Iraq disaster, whose legacy diminishes the party's credibility on national security. That was why he lashed out last week at Perry, who had accused Paul of ignorant isolationism. "Apparently his new glasses haven't allowed him to see the world any more clearly," the latter shot back.

And now his views have drawn the opposition of the prince of the dark side himself, Dick Cheney. Paul was "basically an isolationist", Bush's former vice-president and architect of the Iraq war told ABC TV the other day. To which Paul responds, will the warmongers ever learn? And, he must be privately chortling, with enemies like that, who needs friends?