Try as I might, I still struggle to detect much substance in Barack Obama. Gore Vidal, who has seen Democrats come and go for a very long time, remarked the other week that the senator doubtless has some excellent ideas. The trouble is that he has yet to express any of them, unless you count hope as a novel concept.

Hillary Clinton, on the other hand, has depths. Nor are they hidden. She has been plumbing a few with enthusiasm since it became clear that there was no mathematical possibility of her winning her party's nomination outright. She passed through layers of shamelessness weeks ago, and is still digging.

As a psychological study, it makes for an interesting case. Anyone who thought they had the measure of the Clinton machine when Bill faced impeachment has had to think again. Either Mrs Clinton is attempting to convey the idea that she is actually unstoppable, and will not be deterred by a little matter like a shortfall in votes, or she is unable to come to terms with the possibility of defeat. The spectacle is not edifying.

It does not, however, bestow any special virtue on Mr Obama, any more than the "historic choice" of an African-American imposes some sort of obligation on voters. Race, like gender, is an issue that weighs heavily on political life in the United States but, since the candidate refuses to be "defined" by his colour, we should take him at his word. He was at his most eloquent in dealing with the issue after his former pastor inflicted controversy on the campaign. That should be that, at least until the Republicans set to work.

For now, both Mrs Clinton and John McCain prefer a different tactic. The former was a near certainty to win next week's Pennsylvania primary even before Mr Obama attempted to explain why he has difficulty in reaching white working-class voters. The latter - reportedly of the opinion that the Illinois senator is "phoney" - was increasingly convinced that he could deprive either Democrat of the presidential prize. Now, in an alliance you would struggle to call holy, the pair have seized on what they take to be a self-evident weakness in Mr Obama.

Not stupidity; not a lack of decency; not a colourful private life or a dangerously inflated ego. The charge, under American rules of political engagement, is much worse: the charismatic Obama is, they say, an elitist, an out-of-touch liberal who fails to understand ordinary Americans and is therefore unfit to represent them. He lacks, in short, an essential instinctive patriotism. Having apologised for his remarks, Mr Obama has seemed almost to agree.

The remarks themselves, spoken in San Francisco, that haven of decadent liberalism, bear examination. Asked why he has had such a tough time connecting with the white working class - the people who will decide the race in Pennsylvania, and in Indiana on May 6 - Mr Obama said: "You go into these small towns in Pennsylvania and, like a lot of small towns in the Midwest, the jobs have been gone now for 25 years and nothing's replaced them. And it's not surprising, then, they get bitter, they cling to guns or religion or antipathy to people who aren't like them, or anti-immigrant sentiment, or anti-trade sentiment as a way to explain their frustrations."

So here's something remarkable. Judging by the storm that followed, with McCain weighing in and the Clinton camp handing out "I'm not bitter" stickers, such thoughts simply cannot be spoken. Truth or accuracy are irrelevant. Rising unemployment, the sub-prime nightmare, prices careering out of control, the consequences of bloody wars: none of these matter, somehow. On present form, Mrs Clinton is the last person liable to say: "It's the economy, stupid."

Mr Obama has appeared to attack God, guns and the people who keep faith with such things. That's the story; that's sufficient. Neither of his rivals countenance the possibility of admitting that the rhetoric might contain an atom of truth. Instead, Mrs Clinton, her outrage real or simulated, said: "People embrace faith not because they are materially poor but because they are spiritually rich. People don't need a president who looks down on them. They need a president who stands up for them."

This is, as it happens, a quintessential Republican line. It echoes the sort of attacks that were so effective against John Kerry and Michael Dukakis. It does not - for this would be un-American - begin to acknowledge that ordinary citizens might just have one or two reasons for bitterness. Mrs Clinton could have said that the economic woes of the US prove the pressing need to keep the Republicans out of the White House. But not a bit of it.

At the heart of Mr Obama's problems - inevitable defeat in Pennsylvania is only the first - lies a familiar paradox of American politics. How do you talk about class, and the consequences of class, in a society that insists it is classless? No-one seriously doubts that the housing calamity, food prices and joblessness are hitting the least well-off hardest. Mr Obama described the condition of middle America as he saw it, and he did not blame anyone for feeling bitter.

Mr Clinton said, nevertheless, that his remarks were "demeaning". The Republicans instantly packaged the episode as the "blunder" they had been waiting for. Fair enough, if that's your taste. You might wonder, nevertheless, about the condition of a political debate in which possible truths cannot even be mentioned.

In 1987-9 the wealthiest 5% of American families were paid nine times as much as the poorest 20%. By 2004-6, thanks to George Bush's tax cuts, booming financial markets and the effects of world trade, the rich were earning 12 times as much as the poor. According to the Economic Policy Institute and Centre on Budget and Policy Priorities, even the "middle fifth" have fallen behind, their incomes rising by 13% over 20 years, while the top 5% have seen a 60% increase. Incomes for the very poorest have actually fallen, by 2.5%, since 2000.

Mr Obama was not peddling a myth, nor was he offering an outlandish insight into the ways in which people react to hard times. He was talking like a wimp liberal, however, supposedly incapable of sharing the pain of ordinary folk, unlike a high-powered lawyer who was once First Lady. He failed, too, to give much credit to the consolations of faith and small arms. But the missing jobs he mentioned? Not worthy of debate.

Mr Obama is not, and never was, some street kid from the hood. Then again, Mrs Clinton is no Rosie the Riveter either. Mr McCain is an authentic war hero, but the sons of admirals do not tend to grow up in the shadow of a Pennsylvania steel mill. Mr Obama said nothing that was original, and nothing that would surprise a European observer. He tried to describe a truth, and was hammered for his pains. For him, Pennsylvania was long a lost cause, but whether the ruthless Mrs Clinton deserves the prize is another matter.

Not my business, of course. When the storm has subsided and liberalism has once again been vanquished, however, perhaps America's voters will risk a question. How would Mrs Clinton or Mr McCain set about salvaging the economy of the US if they cannot recognise the plight of its ordinary citizens?