There is no group of Americans as revered by politicians and pundits as "the middle class." Pity, then, that no one actually knows who is in the middle class.

We can define the poor, or at least something close to it: it's individuals and families living below the official poverty line, which was, in 2014, $11,670 for a single adult and $23,850 for a family of four.

But there's no analogous "middle-class line." In the New York Times, Dionne Searcey and Robert Gebeloff try to create one, defining "middle class" as households making more than $35,000 and less than $100,000. Using this definition shows that the ranks of the middle class have been thinning for decades. But you can see Searcey and Gebeloff preparing for the backlash. "Although many Americans in households making more than $100,000 consider themselves middle class, particularly those living in expensive regions like the Northeast and Pacific Coast, they have substantially more money than most people."

As that caveat suggests, the amount of money needed to feel middle class varies sharply across the country. Making $50,000 leaves you struggling in Manhattan and wealthy in Detroit. Some argue that our definitions of "middle class" and "wealthy" should adjust for local living costs. But maybe that's an adjustment too far: is living in New York City a necessity, or is it a luxury good, much like buying a fancy car or a huge house?

The angst over local-living costs gets to a deeper problem. Middle class is, in some ways, a state of mind, or at least a state of comfort. The Times quotes Christine L. Owens, executive director of the National Employment Law Project, offering her definition of middle class, which includes no income threshold at all: "I would consider middle class to be people who can live comfortably on what they earn, can pay their bills, can set aside something to save for retirement and for kids in college and can have vacations and entertainment." But what does "comfortably" mean? What kids of vacations are included?

And then there's the efforts to slice the salami yet thinner. A December New York Times poll found that 38 percent of Americans considered themselves middle class. But almost as many — 33 percent — considered themselves "working class". And another 12 percent thought they were "upper-middle class". We don't just lack for a clear definition of middle class. We lack for a clear definition of its closest relatives, too.

But the definition matters. Every politician wants to appeal to the middle class. President Obama's 2015 State of the Union pushed America to embrace "middle-class economics," which he defined as "the idea that this country does best when everyone gets their fair shot, everyone does their fair share, and everyone plays by the same set of rules." In truth, the policies the White House packaged under middle-class economics would do quite a lot for the poor — but they are designed so that they will also do a lot of people who are not poor. They will even help some people who are rich.

That's one of the problems with middle-class creep. America is an odd society in that almost no one wants to be considered rich. In that Times poll, only one percent of the country thought themselves "upper class" — perhaps because it carries a whiff of handlebar moustaches and that sad part in the Muppets Christmas Carol before Michael Caine stops being a jerk. But as the definition of middle class grows, the policies needed to target it also begin to stretch.

Take Obama's plan for free community college. It sounds like a proposal meant to help the poor. But as Libby Nelson explains, "for the poorest students, tuition is usually entirely covered by federal financial aid, sometimes with money left over to help pay for living expenses."

So the people who will really benefit are students who aren't poor, but are still cost-constrained: the middle class, in other words. And some of the people who will benefit don't need the money at all. There are good arguments to be made for universalizing these kinds of programs (we do it for K-12 education, after all), but often, the good arguments aren't made: there's just a desperate attempt to target a vague, but increasingly sprawling, venerated class of not-rich-but-not-poor Americans, and in order to get your policy certified as middle-class friendly, it needs to help a huge swath of people, some of whom are doing just fine.

Which is all to say that a politician's idea of where the middle class begins and ends can powerfully shape the way policy actually looks. Pity, then, that we don't have a clearer definition of this all-important political category.