It's more than 30 years now since Ronald Reagan asked the most potent question a challenger can ask when seeking to defeat an incumbent president: "Do you feel better off today than you did four years ago?"
Last night, Mitt Romney, in accepting the Republican party's nomination as challenger to Barack Obama, asked the same question -- knowing full well that for many American voters, in the midst of a prolonged economic slow-down, the answer is a resounding No.
Remember Sarah Palin, in that brief moment when it looked as if she might become the Republican party's standard-bearer? She used to ask the same question in a folksier, but perhaps even more potent, way: "How's that hopey-changey thing working out for ya?"
Because of course hope and change is exactly what Barack Obama did offer four years ago -- and for many American voters, the hopes of 2008 have become the disappointments of 2012.
As Mitt Romney put it last night: "Hope and change had a powerful appeal. But ... if you felt that excitement when you voted for Barack Obama, shouldn't you feel that way now that he's President Obama? You know there's something wrong with the kind of job he's done as president when the best feeling you had was the day you voted for him."
The US is still, despite everything, the biggest economy in the world. The man who sits in the Oval Office makes decisions that reverberate far beyond the US's shores. That's why, every four years, some people outside the US ask: "Shouldn't we all get a vote?"
The US opinion polls suggest that this year's election will be a close one. It's rare for presidents to be defeated after serving just one term (Jimmy Carter in 1980 and the first President Bush in 1992 are the most recent exceptions), but no one is taking it for granted that Barack Obama will still be in the White House next January.
It would be a different story if the rest of the world did have a vote, although it's true that national leaders are often far more popular overseas than at home (Margaret Thatcher was the prime UK example). According to one recent poll, 87 per cent of German voters, 86 per cent of French voters, 80 per cent of British voters, and 74 per cent of Japanese voters have confidence in Obama -- and large majorities want to see him re-elected.
Part of Mitt Romney's appeal to American voters is that he will be a tougher President than Obama. He believes, as he put it last night, that "when the world needs someone to do the really big stuff, you need an American."
He didn't mention Afghanistan or Iraq, but he did mention Iran, and he warned Russia and China that he'd be tougher on them too. (I couldn't help noticing, by the way, that he didn't once mention the last Republican president, George W Bush, who seems to have been almost entirely written out of the Republican history books, at least for now.)
It's often said that all that matters in any election is how voters feel about the economy -- and more specifically, about the economic future. In fact, it's more complicated than that, which means that despite the grim economic picture -- and in particular the jobs picture -- Barack Obama is still in with a chance.
For one thing, as Hillary Clinton learned during the Democrats' primary campaign four years ago, he's a supremely effective campaigner. He's also an inspiring speaker, which no one would claim for Mitt Romney. (Mind you, it may be that in 2012 there's less of an appetite for inspiration, and more of an appetite for perspiration.)
The electoral demographics may also be in Obama's favour: women voters, African-Americans, Hispanic voters, all lean towards the Democrats -- and the recent furore over a Republican congressman's remarks about "legitimate rape" will have done nothing to help Romney.
Next week the Democrats will hold their own convention, and after that, it'll be time for the televised presidential debates. I'll be watching, of course, and I'll also to be doing some reporting from the US in the run-up to the election in November.