On Independence Day weekends in presidential election years, the ritual is normally immutable. The candidates tour the heartland, dutifully down their hot dogs and hamburgers, and, on this most patriotic of national holidays, declare their undying love for America and all it stands for. The country, meanwhile, gets on with the barbecues, parades and fireworks, and pays not a blind bit of attention.
This time, though, there's a difference. In American politics, patriotism is the most potent of issues – and in 2008, a number of Americans seem to harbour some doubt over whether the love of the US on the part of one candidate is quite as wholesome and undying as it should be. Which is why Barack Obama, Democrat and possibly the most liberal member of the US Senate, has spent the past week on a mission to plant his own flag in the safe, indisputably patriotic, political centre.
His travels have been as instructive geographically as ideologically. First was Independence, Missouri, heartland home town of Harry Truman, that most down-to-earth of American Presidents. There, Mr Obama proclaimed his "deep and abiding love for this country". Then he was in Ohio, talking up faith-based social service, an issue so dear to America's Christian "moral majority".
Then on to Colorado Springs, a town that lives and breathes the US armed forces. In case anyone missed the point, the next day in North Dakota he was talking up the need to look after America's veterans better. Rounding things off was Independence Day itself – 4 July, which he chose to spend in Montana, "Big Sky Country" and symbol of the mythical Old West, so tightly bound up with America's image of itself.
Mr Obama's precautions are wise. Dr Johnson described patriotism as the last refuge of the scoundrel; in the US it is the first resort of the Republicans come election time.
The strategy is not new. Back in the 1950s, candidate Adlai Stevenson rashly ventured that patriotism was about loving America, not hating the Russians. For his pains he was branded an appeaser (much as Mr Obama is now for having offered to talk with the Iranians) and was twice thrashed at the polls by Dwight Eisenhower. George McGovern might have been a Second World War veteran, but unlike his 1972 opponent Richard Nixon, he failed to wear a flag pin in his lapel and lost by a landslide. In 1988, the patriotism issue helped doom Michael Dukakis, as the Republicans seized on the fact that, as Governor of Massachusetts, he had opposed a measure making the pledge of allegiance obligatory in state classrooms.
September 11th only upped the stakes. George W Bush played the card ruthlessly in the 2002 mid-term elections, implying that anyone (like Mr Obama) who opposed his impending invasion was unpatriotic. Two years after that the infamous "Swift Boat" campaign managed to turn decorated Vietnam war hero John Kerry (below) into a slippery agent of influence for foreigners – even though Kerry had voted back in autumn 2002 in favour of attacking Iraq.
In every case, there are two common threads. Each victim was a Democrat, targeted as a liberal, and therefore "un-American". And each time, the Democrat lost. In a year when they were inevitably going to face a desperate fight to hang on to the White House, and when their man John McCain, tortured prisoner of war in Vietnam, is the embodiment of patriotism, you could bet a thousand stars to a stripe that they would try it again.
Mr Obama hasn't always helped his cause. For a long while he not only did not wear the magic flag pin, but in no uncertain terms explained why. "Shortly after 9/11," he declared last year, and particularly over the Iraq war, the pin "became a substitute ... for true patriotism, which is speaking out on issues that are of importance to national security". True patriotism, he said, was to set out the right thing for the country to do.
Herein lies a crucial difference between Republican and Democratic, or "liberal", concepts of patriotism. The former comes close to asserting that "My country, by definition, can do no wrong." Less respectful of the past, the liberal version looks to the future.
Mr Obama is vulnerable, however, on both counts. There were the anti-American diatribes from the pulpit of his minister, the Rev Jeremiah Wright, whom the candidate was forced to disown. Then Michelle Obama remarked that her husband's candidacy had "for the first time" made her feel proud of her country.
Permeating everything is his exotic background – his Kenyan father, his Indonesian stepfather, and part of his childhood spent in the world's most populous Muslim country. A further complication is his race, whose true impact will be measured only when the returns flow in on the night of 4 November.
One thing, though, is clear. If patriotism helped to scupper John Kerry, what might it not do to an African-American – whom one in 10 Americans, the polls say, thinks is a closet Muslim? Whether or not the McCain campaign is behind the rumours is beside the point. It's up to Obama himself not to repeat Kerry's mistake in 2004, when he let the patriotism poison fester. He has to fight back – and he is.
In speeches, Obama takes on the whispering campaigns head on. His staff have also set up a website, fightthesmears.com, designed to scotch untrue allegations – such as the supposed existence of a tape in which Michelle denounces "whitey". Last but not least, and like every candidate in every general election, he is moving towards the centre. The flag pin is now firmly back in his lapel. To the dismay of many liberals, he is taking more conservative positions on issues such as guns and warrantless government wiretapping of terrorism suspects. He even hints that his former 16-month deadline for troop withdrawal from Iraq is no longer set in stone.
Inevitably, the dreaded cry of "flip-flop" now reverberates through the land. Mr Obama, the commentators say, is just another opportunist candidate, ready to say what pleases the immediate audience, regardless of what he said before; how can he be trusted when in office? To which, in the case of Iraq, he may reply to his critics as did John Maynard Keynes, when attacked for shifting his position on government intervention in the economy. "When the facts change, I change my mind. What do you do, sir?"
More to the point, flip-flopping is a rite of presidential election summers. McCain, too, is flapping around like a live trout on a fishmonger's slab. You could almost say it's patriotic.