Six in ten Americans claim to be part of it, economists squabble over who is in it, and later Tuesday Obama will become the latest American politician to tether his electoral fate to it.
Like the US Supreme Court justice's infamous benchmark for pornography in the 1960s, America's middle class is tricky to define but you know it when you see it.
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"There isn't really a standard definition," explained sociologist Scott Winship of the Brookings Institution, "when you ask people, usually 50 to 60 percent say they are middle class."
That is dubious. If middle class is defined by having an income of plus or minus half of the median -- around $49,445 a year -- then the group makes up only 40 percent of the population.
"It is a diverse group, you have a lot of people, who are young, whose incomes are quite low, but who eventually will be doing quite well," said Winship.
"You have a lot of folks who you might call rich, but who like to think of themselves as middle class."
Amid the jumble, politicians -- even president Herbert Hoover who declared the United States to be a fluid classless society -- have to make sense of this amorphous mass, often the key to notoriously tight US elections.
In the last three presidential elections the winner has been backed by an absolute majority of people earning between $30,000 and $99,999 dollars.
Obama was ushered into the White House with roughly 51 percent of that vote in 2008, compared to John McCain's 47 percent according to exit poll data.
Obama's win among the middle class was much more pronounced in battleground states that helped hand him the presidency: Colorado, Ohio, Iowa and Virginia.
During the 2004 election, when Democrats were soundly beaten, John Kerry got 47 percent of the middle class vote whereas Republican George W. Bush got over 53 percent.
In November's election Obama is hoping that the middle class will be wooed by his promises to reverse the losses seen since the financial crisis and the Great Recession.
Between 2000 and 2010 median income has fallen seven percent, millions are unemployed and the housing bubble has ravaged middle class wealth.
"This is a make or break time for the middle class," senior Obama advisor Valerie Jarrett told CNN Tuesday, laying out the populist economic pitch that forms a key plank in Obama's reelection platform.
Obama is betting that his multi-millionaire rival Mitt Romney -- if he wins the Republican nomination -- will fail to connect with suffering middle class voters, leaving his full-throated defense of capitalism ringing hollow.
"Obama is very clearly thinking that the electorate will respond to a message that pits the middle class, against the top," said Winship.
But populist Obama's gambit is made perilous by the fungible nature of the middle class, and its All American ethos that with hard work and guile anything is possible.
"There is no doubt that everyone in America has struggled because of the crisis, you have definitely seen that median income levels have dipped because of what we have seen over the past couple of years," said Tess Stovall of the Third Way think tank.
"But the middle class remains optimistic, remains aspirational, they still want to send their kids to college, they still want to plan for a successful retirement."
"When you poll typical middle class Americans about their outlook over the coming year they say we think it is going to get better. They feel like they're doing better than most, but they're concerned about the future of the country as a whole."
As Great Depression-era writer John Steinbeck put it: Poorer Americans see themselves as "temporarily embarrassed millionaires" rather than the exploited proletariat.
So much for the middle class.
