A remarkable role reversal has occurred in US politics. The wimps have become the he-men. Or more precisely, the Democrats have taken over from the Republicans as the party Americans most trust to keep the country safe. Not since the days of JFK has this happened, and the transformation could just tip the outcome of November's election.

Last week was, of course, the perfect time for Barack Obama to showcase his achievements. It saw the anniversary of the killing of Osama bin Laden and a dramatic visit to Kabul, during which he delivered a prime-time speech to his compatriots, 7,000 miles away, on how he would end the unpopular Afghan war. According to every poll, the public believes, for the first time since the Vietnam War, that the Democrats are the best bet on foreign policy and national security.

The Republicans' reputation was basically made by Nixon and Kissinger, while that of the Democrats was further tarnished by the 1979 Soviet invasion of Afghanistan and the Tehran hostage crisis, both on Jimmy Carter's watch. Carter's successor, Ronald Reagan, is universally credited with strength and statesmanship in his dealings with Moscow. The end of the Cold War, and the masterfully executed war to drive Iraq from Kuwait, came under Reagan's successor, George H W Bush – to this day, the embodiment of a Republican safe pair of hands.

The Democrats, meanwhile, were all over the shop. The image of diminutive Michael Dukakis, the party's 1988 nominee, with his helmeted head poking out of a battle tank, ranks among the most disastrous photo-ops in history. Dukakis as commander-in-chief? No thanks. Four years later, Bill Clinton did win, but his flabby salute, his initial focus on gays in the military and the "Black Hawk Down" humiliation in Mogadishu quickly stamped him as an amateur in national security matters. When he did act tough in 1998, by bombing Iraq and alleged terrorist sites in Africa, it was widely seen as a diversion from the unfolding Monica Lewinsky affair.

Indeed, by the time the terrorists destroyed the World Trade Center on 9/11, even liberals were prepared to concede that George W Bush's theft of the 2000 election might have been a blessing in disguise. With Colin Powell at the State Department and Don Rumsfeld at the Pentagon, it was said, at least grown-ups would be in charge during the tumultuous period ahead.

As the Iraq debacle proved, such confidence was misplaced. But the Republican mystique lingered long enough to help Bush to re-election in 2004. Since then, it's been downhill all the way.

In 2008, John McCain, a war hero, threw away that advantage by choosing Sarah Palin as his running mate. And this time around, the Republican primary campaign has plumbed new depths of fatuity, featuring the likes of Herman Cain (he of "Ubeki-beki-beki-beki-stan-stan") and Newt "the Palestinians are an invented people" Gingrich. Most candidates sought only to outdo each other in unthinking hawkishness, berating Obama for his failure to "stand up for America".

In this festival of infantilism Mitt Romney came across as a comparative adult. But, like the Bourbons, his party seems to have learned nothing and forgotten nothing. Who, for instance, might be a Romney secretary of state? The name most often heard is that of John Bolton. Yes, Bolton whose hawkishness made even Senate Republicans queasy when Bush wanted to make him UN envoy in 2005.

All of the above left a huge opportunity for Obama. His first clever move was to persuade Bob Gates, that emblem of Republican competence, to stay on as Secretary of Defense. But that would have counted little had he not shown he was up to the job.

Make no mistake, this Democrat is anything but squeamish over the use of US power. Obama may have let Europe be the visible leader in Libya and wound down the war in Iraq. But in December 2009, against the advice of many advisers, he ordered a surge in Afghanistan, and has used drones against terrorists, infringing Pakistan's sovereignty in the process. And then, of course, there's Osama bin Laden. If Obama is to be pigeonholed, it is as a hawk – but a common-sense hawk. In this, he has been nothing if not consistent. "I don't oppose all wars... what I oppose is a dumb war," the then Illinois state senator declared in October 2002, as George W's legions massed on Iraq's borders. That remains his basic approach. The real surprise is that the role most comfortably assumed by a young first-term senator with no executive experience is that of commander-in-chief. He comes across as he is: cool and analytical, yet ready to take a risk (when he authorised the Seals to take out Bin Laden, he overruled both Gates and his vice-president).

Obama's decisiveness only underlines the paradox of US elections. Usually, the fuss is about domestic policy – above all, the economy, where a president's power to change things is limited. Foreign policy counts for little, but it is where, for better or worse, he has virtually free rein. This time, it could be different: the economy may yet decide the outcome in November. However, in a close race, perceived foreign policy competence could make the crucial difference – in favour of the wimps turned he-men.