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The phrase "latte liberal" is a familiar pejorative in the US. What is it about milky coffee that people associate with left-of-centre politics?

It is light and frothy, say conservatives. Foreign-sounding. An expensive indulgence. They don't think much of the coffee, either.

In the tribal discourse of US politics, the espresso with steamed milk is identified squarely with one side. It is the warm beverage of progressivism - more specifically, that proverbial subset of affluent, coastal-dwelling, Prius-driving intellectuals whom talk radio hosts like to lambast.

The term "latte liberal" applies, according to the website Urban Dictionary, to those "who sit around and drink overpriced diluted Starbucks coffee while lamenting the plight of the poor".

Republicans, presumably, prefer filter.

Liberals (and coffee-lovers) might well protest that this stereotype is crude and unfair. But there is little doubt that it's widely understood.

When Barack Obama was criticised for hailing US Marines while holding a coffee cup, the incident quickly became known as the "latte salute". No matter that the contents of the receptacle were never disclosed. It was assumed that a Democratic former college professor would drink nothing else.

Image copyright Getty Images Image caption Vice President Joe Biden, coffee icon

New York Mayor Bill Di Blasio was mocked for his "small soy latte liberalism" by Republican former House Speaker Newt Gingrich. The former had told his city's wealthiest citizens that a planned tax rise would cost them the same as one miniature protein-based drink each day.

Most famously, an attack ad by the fiscally conservative Club for Growth during the 2004 Democratic Iowa caucuses urged candidate Howard Dean to take his "tax-hiking, government-expanding, latte-drinking, sushi-eating, Volvo-driving, New York Times-reading, body-piercing, Hollywood-loving left-wing freak show" back to his home state of Vermont.

It helps that the words "latte" and "liberalism" alliterate.

But the power of the association lies in its suggestion that liberals are elitist and out of touch. Just as the UK has its champagne socialists and France its gauche caviar, American conservatives have sought to mock affluent progressives as ineffectual intellectuals who don't have to live with the consequences of the policies they espouse (by contrast, "Nascar-watching" and "Bible-bashing" are adjectives frequently used to dismiss Republicans).

Richard Nixon accused Adlai Stevenson of being an "egghead" in 1952. Segregationist Alabama Governor George Wallace characterised his critics as "pointy-headed college professors". The term "limousine liberal" was first deployed in 1969 by New York Mayoral candidate Mario Procaccino, a conservative Democrat, to describe his rival, the progressive Republican John Lindsay.

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But in a consumer age, food and drink are especially potent signifiers of lifestyle. As far back as 1937, George Orwell bemoaned the "bearded fruit-juice drinkers who come flocking towards the smell of 'progress' like bluebottles to a dead cat".

Until the mid-1990s middle-class US liberals were typically associated with a fondness for brie and chardonnay, according to Geoffrey Nunberg, a linguist at UC Berkeley School of Information. "It's always a white food and a soft food," he says.

The term "latte liberal" came to prominence when it was used in a 1997 article for the Weekly Standard by the conservative writer David Brooks. He described "latte towns" in which "liberalism is a dominant lifestyle as well as the unchallenged ideology, and where social concern takes the form of concrete activism".

Very quickly, the phrase caught on. "It explodes. It becomes a standard term," says Nunberg - the subtitle of whose book Talking Right references the anti-Dean ad. "This is a white middle-class lifestyle that people associate with liberalism."

For one thing, the boom in coffee chains was closely identified with liberal urban areas such as Seattle, where Starbucks is headquartered, San Francisco and New York.

Its European name (from the Italian caffe latte) carried the hint that its consumers were effete and vaguely un-American. Coffee shops are places where people sit around talking rather than getting anything done.

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The fact latte is relatively expensive also carried the hint of hypocrisy - the implication being that the socially concerned latte-drinker should be donating his or her spare cash to the poor rather than frittering it on expensive hot drinks.

Helpfully for political opponents, it also sounds mushy and insubstantial. "Latte is soft and white and runny," says Nunberg.

And yet at a time when there is seemingly a Starbucks on every street corner and latte is sold in truck stops, there is something rather archaic about representing latte as the beverage of the elite.

Instead, the reality is that "it's a form of industrially produced, high-calorie, low-cost, high-profit, non-nutritious food", says Kyla Tompkins, an expert in food studies at Pomona College, California. While it's true, she says, that there is cultural capital attached to knowledge of products like coffee, latte's core consumers are sleep-deprived working class people who use it as a stimulant. Likewise, the baristas who serve lattes are, typically, not exactly high-spending elitists.

And the empirical evidence that it is the preferred drink of those on the liberal end of the political spectrum is scant.

During the 2008 elections, forecaster Nate Silver ranked US states according to a "Starbucks: Walmart Ratio". The more branches of the coffee chain compared with those of the supermarket giant, Silver found the more statistically likely a state was to vote for Obama over John McCain. But it was never established that a preference for latte over, say, Americano had any psephological significance.

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In a forthcoming paper titled Why Do Liberals Drink Lattes? for the American Journal of Sociology, Michael W Macy of Cornell University studied whether political ideology correlated with lifestyle preferences listed in the General Social Survey. "Unfortunately, it didn't include hot beverage preference," says Macy. While two-thirds of those items listed did indeed have an association with political outlook, it was impossible to say if latte-drinking was among them.

The election cycle of 2004 represented the high water mark of the "latte liberal" jibe, says Nunberg. Not only was there the Club for Growth's memorable attack on Dean, the presidential race eventually pitted George W Bush - widely portrayed as a Texan everyman, despite his wealthy background - against Democrat John Kerry, who frequently attracted conservative mockery for his personal fortune, aristocratic bearing and love of windsurfing. If ever there was a latte liberal, the Bush campaign appeared to suggest, here it was.

But Nunberg argues that this line of attack had subsided by 2008. "Sarah Palin was the last populist class warrior the Republicans produced," he says. At the 2012 election, in a reverse of the treatment dished out to Kerry, it was Republican Mitt Romney who was attacked for his alleged elitism, owing to his personal wealth and his comments about "the 47%".

Perhaps even more significantly, latte has been around long enough to no longer be seen as an especially exotic item. "McDonalds is selling five varieties of mocha now," says Nunberg. Almost everyone in the US is a latte liberal, or conservative, or independent, these days.

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