On the face of it, foreign and national security issues play into the hands of Mr McCain, a Vietnam war hero with decades of service on the influential Senate Armed Services committee, of which he is now the ranking Republican member, and the embodiment of the "experience" voters say they seek in a president.
Republicans have built a generation of political dominance on the perception they are better equipped to deal with America's enemies and "keep the country safe". A reminder came last month, when the crisis over Russia's invasion of Georgia, and Mr McCain's forceful response, contributed mightily to his pre-convention rise in the polls. His choice of the foreign policy novice Sarah Palin as running mate may have been mocked. But it barely dented his own credentials.
The Wall Street meltdown and the tottering economy are playing to a key strength of the Democrats and Obama, and are the main reasons why the latter appears to have regained a lead in the polls. But a new crisis abroad could upset everything.
"Foreign affairs – the Russia-Georgia situation, or Iran, or North Korea, or any of these issues – could explode and be front and centre within a matter of days," says Neil Newhouse, a top Republican pollster. "It would be like the Lehman Brothers issue just happened, and the campaign's got to respond to it."
What happens over the next six weeks is anyone's guess. But an unforeseen event could tip the balance, as happened in 2004, when an Osama bin Laden diatribe against the US, on the final weekend of the campaign, is believed to have helped, or even sealed, victory for George Bush.
With a record amounting to a couple of years on the Senate Foreign Relations committee before he began his White House campaign, Mr Obama cannot measure up. But he may be less vulnerable on national security than he at first appears.
For one thing, President Bush's foreign policy has become far less confrontational. Not only have distinctions between the parties become increasingly blurred. Between Messrs Obama and McCain, the differences are far less stark than a year ago.
A more stable Iraq has quieted the debate on a timetable for withdrawal of US troops, which Mr Obama promises by late 2010. Both men favour a pull-out, but not one risking the gains of the surge. During the primaries, the Illinois senator was lambasted by Hillary Clinton for his "naive" willingness to sit down and talk with Iran, Syria and North Korea without pre-conditions. Now even the Bush administration is edging towards doing just that.
Both he and Mr McCain agree that Iran should not have nuclear weapons, and have condemned Russia's aggression. Both want a shift in US focus and resources to the war against the Taliban and al-Qa'ida in Afghanistan and Pakistan. Again, Mr Bush has just announced steps in that direction. Both are staunch supporters of Israel – in Mr Obama's case to the point of stating that Jerusalem would remain the country's undivided capital.
But their world views, and basic approaches, are very different. Mr McCain is a hawk. He was a supporter of the Iraq war and advocated the surge in US troops. Like Mr Bush, he sees the US at threat in a global war against Islamic terrorism, and proposed a "League of Democracies" to carry the fight forward.
He has often been his own top foreign policy adviser. Those closest to him are split between realists and neo-conservatives. The former include that supreme practitioner of realpolitik Henry Kissinger. The latter include founder members of the "Project for the New American Century".
Mr Obama takes a more nuanced view that gives an unparalleled chance to understand what other nations think about the US, as opposed to how the US imagines others see it. He talks of defeating al-Qa'ida – but far less of an ill-defined, unending "war on terror". His advisers tend to be young, though seasoned by veterans like Zbigniew Brzezinski and Tony Lake. More typical is Susan Rice, 44, a former assistant secretary of state tipped to be Mr Obama's own national security adviser if he wins.
John McCain's foreign policy at a glance...
Iraq
McCain was a strong backer of the surge and now claims success for the strategy. He also supported the invasion. He refuses to set a withdrawal timetable, but says he will not keep US troops in Iraq for "a minute longer than necessary".
Iran
Will continue the Bush policy of undermining the clerical regime through internal dissent. He wants "real pressure" put on Iran over its nuclear programme and would keep alive a military threat. He also wants to press Iran to end its "interference" in Iraq.
Afghanistan
McCain backs the same military strategy in Afghanistan that "succeeded" in Iraq, so would send more troops and encourage Nato members to contribute more soldiers.
Russia
McCain was a harsh critic of Russia's invasion of Georgia and called for it to be thrown out of the G8 in punishment. Sarah Palin, who has no foreign policy experience, has said: "They're our next-door neighbours and you can actually see Russia from land here in Alaska." That comment, and the fact that she only got a passport last year, got the ticket into trouble yesterday.
... and Barack Obama's
Iraq
The Illinois senator opposed the Iraq war. He also opposed the troop surge and called for a withdrawal timetable, which has received the broad backing of the Iraqi government. But his running mate, Joe Biden, has caused a stir with his proposal for Iraq to become a loose federation split into three parts – Shia, Sunni and Kurd.
Iran
The Obama/Biden plan favours dialogue to solve the trickiest foreign policy challenge for the incoming administration, with Iran continuing its nuclear programme. Obama intends to work with international allies to isolate Iran diplomatically and still supports "tough, direct presidential diplomacy".
Afghanistan
Obama as president would shift troops from Iraq into Afghanistan, which he considers to be the real terrorist threat alongside Pakistan.
Russia
Obama was caught out by the Russian invasion of Georgia and only hardened his rhetoric after initially calling for restraint. In the longer term, he hopes to work with Russia on nuclear disarmament.
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