I'm not sure when the term latte liberal replaced the old champagne socialist as the favoured term of derision for the well-heeled leftie but it looks an increasingly useful metaphor for understanding how the deadlock in the Democratic presidential primary election might be broken.

The two candidates have fought themselves to a standstill. In the closest race in any US presidential primary campaign in decades, Hillary Clinton and Barack Obama are more or less tied in total votes received and in delegates elected for the party's nominating convention.

Super Tuesday, when almost half the country voted in the nearest thing ever to a nationwide primary, was supposed to break the logjam but has merely tightened it.

The reason the race is so close has nothing to do with policy differences. I'd wager that not one voter in a hundred could name with any confidence a single difference between the two candidates' stances on the war in Iraq, healthcare, taxes, public spending, abortion or anything else. That's because there isn't one.

The fault lines in the contest instead fall largely along differences in identity - ethnic and gender - and values. Mr Obama and Mrs Clinton have, as we have noted before, both established massive, almost identically sized coalitions of voting blocs aligned along these cleavages.

Mrs Clinton wins heavily among white women, older voters and Latinos. Where they voted in large numbers on Tuesday, she won by large margins.

Mr Obama won states where his following of younger voters, African-Americans and white men predominated.

But one other critical factor - the one that may ultimately determine who wins this race - is whether the voter is sinking or swimming in the modern economy.

Mr Obama wins disproportionately among people who may be considered the winners in the global economy: the well educated, the mobile and the financially secure. Mrs Clinton's voters are the strugglers, the class that feels itself left behind by an increasingly unfair global economic system.

Consider the exit poll from California, the largest state to vote on Super Tuesday. Mrs Clinton's largest single demographic voting bloc was those who did not complete a high school education, where she won 82 per cent, against just 15 per cent for Mr Obama. The more educated you became - from high school drop-out, through high school graduate then some college, college graduate and finally postgraduate - the more likely you were to vote for Mr Obama. The only category he won, in fact, was the propeller heads with postgraduate degrees.

Income was another crucial determinant of whom you voted for: 59 per cent of those earning below $50,000 went for Mrs Clinton against 33 per cent for Mr Obama. The only broad income category won by Mr Obama was the top one - more than $100,000. (Intriguingly, in California there was one exception to this rule. The super-earners - those earning over $200,0000 - went narrowly for Mrs Clinton. I can only think this was because of all those louche Hollywood types who long for a return of the moral compass of the Clinton years. Jack Nicholson was making calls for Hillary on Tuesday, telling people to vote for her because “she was the best man in the race”.)

The saliency of economics then, is crucial. Those who said the economy was the important issue facing the country went for Mrs Clinton by 20 points. Those who thought Iraq was the main issue chose Mr Obama by five points.

This is where coffee preferences come in. Among voters whose voting choice is not based on identity politics, Mr Obama's supporters are the latte liberals. These are the people for whom Starbucks, with its $5 cups of coffee and fancy bakeries, is not just a consumer choice but a lifestyle. They not only have the money. They share the values.

They live by all those little quotes on the side of Starbucks cups about community service and global warming. They embrace the Obama candidacy because to them he transcends traditional class and economic divides. He is a transformative political figure - potentially the first black man to be president - and is seen as the one to revive America's faith in itself and restore America's status in the world. For these voters the defining emotion is hope.

Mrs Clinton is the candidate of what might be called Dunkin' Donut Democrats. They do not have money to waste on multiple-hyphenated coffee drinks - double-top, no-foam, non-fat lattes and the like. Not for them the bran muffins or the biscotti. They are the 75-cent coffee and doughnut crowd. For them caffeine choice doesn't correlate with their values but simply represents a means of keeping them going through their challenging day.

Though they don't doubt that global warming is important, they think it can wait. They want to make sure first they can pay the heating bills. They're not in favour of the Iraq war but neither are they so focused on restoring America's

image in the world. They're not necessarily racist, it's just that they're not especially animated by the idealism represented by the first black president. For them anxiety, not aspiration is the defining factor.

So who prevails? That may well depend on the state of the economy. The more voters worry about it and the less they focus on ideals, the better Mrs Clinton's chances. For her, bad news is good news.

As it happens, the latest figures out this week suggest the US is now very probably in recession. Unemployment is rising, house prices are falling, stock prices are slumping, spending is fading, confidence is sagging. There's a whiff of panic in the air. Last week the Federal Reserve cut interest rates by more in the space of eight days than the European Central Bank has done in its entire existence.

People are trading down from Starbucks to Dunkin' Donuts. These may not be the best circumstances for Mr Obama's soaring rhetoric of hope in the future. His hope has to be that things do not get so bad that fear overwhelms it.

In 1992 Bill Clinton rode to an election victory under the slogan, “The economy, stupid”. Sixteen years later, we could say, given the apparent inevitability of a recession and given Mrs Clinton's strong following among the less well educated in American society, that it is an even more fitting message for his wife.

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