In American history, populism has a specific meaning: It’s our non-Marxist way of talking about class. Being a populist means standing up for the little guy against ruling elites.
Such vague claptrap is what Beinart deploys as a definition of populism for exclusionary purposes. And it fails on the face of it. In modern poli-sci terms populism is best defined as "an ideology which pits a virtuous and homogeneous people against a set of elites and dangerous ‘others’ who are together depicted as depriving (or attempting to deprive) the sovereign people of their rights, values, prosperity, identity and voice." 
This is a clear definition of populism that fits well the assorted populist movements in American history, including the current Tea Party movement. Beinart doesn't want to employ it because it specifically negates the rhetorical pretzel logic he has conspicuously constructed to steer the definition back into the fallacious trap of using "populism" as a mere sneering pejorative when describing any popular non-leftist movement. In Beinart's world, ONLY leftist movements employing identity politics can truly be "populist."
Beinart has substituted "little guy" into the equation to exclude anyone falling outside the heroic leftist totem of "the oppressed" from being capable of being "populist." In Beinart's world, only "the oppressed" can be "populists," and in leftist terms that automatically excludes anyone not favored by, well, leftists. It's simply a new polish on the old "victimhood" routine, one intended to deny victim status to anyone not of Beinart's tribe. One is either a victim or an oppressor, and by defining populism in terms of class struggle (while claiming he's being "non-Marxist" in doing so) Beinart seeks to automatically and categorically label the Tea Party people as elitist oppresssors. Thus anyone they are opposed to must categorically be the oppressed.
Now let's watch the second part of the framing attempt, in which Beinart casts the Obama administration in the role of the oppressed, since those opposed to the oppressed by Beinart's definition cannot be populists:
The Tea Partiers aren’t standing up for the little guy; they’re standing up to the little guy. We’ve long known that their leaders, like Sarah Palin, opposed against real regulation of Wall Street. Now we learn that what the Tea Partiers dislike about Barack Obama’s economic policies is that they don’t do enough for the rich. According to the Times, Tea Partiers are more likely than other Americans to think Barack Obama’s policies favor the poor, and they’re mad as heck about it.
Yeah, right. This is merely an attempt by Beinart to cast anyone opposed to the actions of the administration as being part of a privileged racist/elitist mob. Beinart is asserting here that being opposed to the massive growth of federal government and economy-crushing spending by same somehow automatically makes Tea Party people privileged elitists oppressing the "little guy." The "little guy" in this case being the federal government as personified by Barack Obama! (Try oppressing the IRS next time you're summoned for an audit. You'll quickly find out who the "little guy" really is.)
Pretty disingenuous stuff coming from a left-wing elitist Ivy League product of exclusive private schools, Yale, and Oxford. Perhaps Mr. Beinart has mistaken his own wealth of elitist privilege as being an acceptable substitute for intelligence, when even the most cursory examination of his premises makes clear that his ham-handed rhetorical framing is pretty poor carpentry indeed.
As George Orwell once famously remarked, "One has to belong to the intelligentsia to believe things like that; no ordinary man could be such a fool."
UPDATE: Right on cue, here comes reliable tool and left-wing elitist Ivy League product of exclusive private schools, Harvard, and Oxford E.J.Dionne promoting the narrative. Why, you'd almost think it was a coordinated effort on the part of left-wing elitist Ivy League products of exclusive private schools, Harvard/Yale, and Oxford to shape the narrative. Is having had a Rhodes scholarship one of the required credentials for this club? Elsewhere, Glenn Reynolds notes his own powers of prognostication.
UPDATE AGAIN: James Taranto at WSJ's Best of the Web takes notice with "Populism of the Privileged." Heh. Remember that we were there first.
 Albertazzi, Daniele and Duncan McDonnell, 2008, Twenty-First Century Populism: The Spectre of Western European Democracy, New York and London: Palgrave Macmillan, p.3