Donald Trump appears to have graduated from a flash-in-the-pan infatuation to a genuine existential threat to the GOP as we know it.

He's bucked the Republican establishment's preference for immigration reform, coming down hard against undocumented immigration, and even for lowering documented immigration. He's sanguine about hiking taxes on the very wealthy, and he's promised to defend Medicare and Social Security from cuts.

This led reform conservative Ross Douthat to term Trump a "class traitor" in the mold of Franklin Roosevelt. The Week's Michael Brendan Dougherty called Trump "a mortal threat to the GOP," and Damon Linker pointed out that Trump is throwing the party's establishment under the bus in favor of its base.

None of which is wrong. But to fully understand the Trump phenomenon, we need to widen our gaze to the way inequality has remade class politics in America.

For my money, one of the best books on this topic remains political scientist Andrew Gelman's Red State, Blue State, Rich State, Poor State. Roughly speaking, Gelman found that while poor people tend to be socially conservative, they are very economically liberal, and vote based on the latter interest. The upper class and the elite, meanwhile, tend to be more socially liberal and economically conservative, and their voting patterns tend to be more schizophrenic between those two impulses. More importantly, a large minority of the upper class is super right-wing on both social and economic questions.

This class divide has also gotten worse along with inequality. Mid-century, the bottom and top thirds of American earners voted for Republicans and Democrats in equal proportions. Now the top third swings GOP, and the bottom third swings the opposite.

So it's important to realize how American politics "looks" from the top as a result of this. To the top of the food chain, the actual lower class is largely invisible. In fact several studies suggest that whenever the top third to top tenth of income earners disagree with the rest of voters about policy choices, the top third to top tenth gets what it wants without fail. That top third includes the upper-middle class — think soccer moms and small business owners and what Michael Lind called the "local notables." They aren't elites, exactly, but they have little use for the welfare state, they prefer lower taxes on themselves, and they have an interest in keeping the pay and bargaining power of less fortunate workers in check. To the genuine elites, these people often become stand-ins for the "everyday Americans" they clearly are not.

This leads to a crucial takeaway: Not only is the Democratic Party much more diverse in terms of gender, race, culture, religion, and mores, it's much more diverse in terms of class. It enjoys massively disproportionate support from both elites with post-graduate degrees and everyone earning less than $30,000 a year. (Roughly the bottom third of the country.) This means the Democrats have their own problems and contradictions, but their coalition makes at least some room for the lower classes.

The Republicans, by contrast, are an alliance between the upper class and the wealthy elite.

But that alliance has become increasingly unstable, thanks to the Great Recession bringing economic insecurity into the upper third of wage earners. Donald Trump, with all his flare and bluster, has just leapt gleefully into that split to defend the upper class. And within the party, they have more numbers.

So it should be obvious why Trump is a serious contender. The GOP orthodoxy of tax cuts for the elite now and forever is unpopular with the country and a fair portion of the party's own base. Trump certainly won't be able to hike taxes on those making $100,000 to $250,000 a year — he probably doesn't want to — but he can support hiking them on the tippy top with relatively little cost.

The same goes for protecting Medicare and Social Security. Skepticism of these programs resides mainly with the libertarian-ish 1 percent. The older, white voters who make up the GOP base — also members of the upper class — view these programs as benefits they've earned from a lifetime of payroll tax payments. In their eyes, these two programs are morally distinct from ObamaCare, welfare, food stamps, and other benefits showered on the "underserving." As Douthat put it in another post, Trump's supporters probably view themselves as "being squeezed from both sides, by the tax-evading rich and the work-avoiding poor alike."

And there, of course, lies the problem. Trump is no defender of the little guy. It's hard to think of anyone who better represents "the least of these" than the low-income immigrants, documented and undocumented alike, that Trump has it in for. He's also glommed on to the right-wing talking point that disability payments have become a fraudulent payout for Americans unwilling to work. Rather, Trump is a defender of upper class whites who feel the 1 percent looks down on them for being too culturally right-wing and insufficiently cosmopolitan.

When Franklin Roosevelt embraced his role as a "class traitor," it was because he was pushing a genuine socioeconomic revolt: a move to shift the money flow away from the elite and back to everyday workers, starting from the bottom up. Beyond Social Security, the New Deal era brought minimum wage hikes, massively strengthened unions, and a whole host of programs to push towards full employment and to build the middle class. The country's longstanding racial hatreds kept black Americans out of the loop, but in the decades since, the New Deal institutions that survived have opened up to women and non-whites.

A higher minimum wage, stronger unions, and a generally rebalancing of the flow of resources through the economy is in the interests of neither the elite nor the upper class. Whatever their differences, they often agree that everyone beneath them needs to be kept in their place. Trump's rejection of the cosmopolitan elite does not mean his sympathies extend terribly far down the income ladder; he has stepped into a spat that lies almost exclusively within the top socioeconomic third of the country. That this spat is often seen as defining American politics — and that anyone can refer to Trump as a "populist" with a straight face — is an indication of how far inequality has gone in rendering everyone outside that top third invisible to the political discourse.

If Donald Trump is a class traitor, he's the most unambitious and uninspiring one ever.