FOR the last week, liberal journalists have been furiously debating whether a new political correctness has swept over the American left. The instigator of this argument was New York magazine’s Jonathan Chait, normally a scourge of Republicans, whose essay on what he dubbed “the new P.C.” critiqued left-wing activists for their zeal to play language cop, shout down arguments and shut down debate outright.

It will surprise absolutely nobody that I think the phenomenon that Chait describes is real. But I come not to judge but to explain . because whether you like or loathe the “P.C.” label, the rise of a more assertive cultural left is clearly one of the defining features of the later Obama years. This assertiveness is palpable among younger activists, on campus and online; it’s visible in controversy after controversy, from Ferguson to campus rape. And it’s interesting to think about exactly where it’s coming from.

The first source, probably, is disappointment with other forms of left-wing politics. A decade ago, the left’s energy was focused on Iraq; in President Obama’s first term, it was divided between his quest for a new New Deal and Occupy Wall Street’s free-form radicalism. But now the antiwar movement is moribund, Occupy has gone the way of the Yippies and it’s been years since the White House proposed a new tax or spending plan that wasn’t D.O.A.

What’s more, despite all the books sold by Thomas Piketty, the paths forward for progressive economic policy are mostly blocked . and not only by a well-entrenched Republican Party, but by liberalism’s ongoing inability to raise the taxes required to pay for the welfare state we already have. Since a long, slow, grinding battle over how to pay for those commitments is unlikely to fire anyone’s imagination, it’s not surprising that cultural causes . race, sex, identity . suddenly seem vastly more appealing.

The second wellspring is a more specific sort of disillusionment. Call it post-post-racialism: a hangover after the heady experience of electing America’s first black president; a frustration with the persistence of racial divides, even in an age of elite African-American achievement; and a sense of outrage over particular tragedies (Trayvon Martin, Ferguson) that seem to lay injustice bare.

Post-post-racial sentiment is connected to economic disappointments, because minorities have fared particularly poorly in the Great Recession’s aftermath. And this sentiment’s rejection of respectability politics . that is, the idea that the fate of black Americans rests mostly in their own hands . seems to point naturally toward a kind of redistributionism. (Ta-Nehisi Coates’s recent Atlantic essay “The Case For Reparations” made this argument explicitly.)

But again, because the paths to economic redistribution are mostly blocked, the more plausible way to put post-post-racialism into practice is social activism: a renewed protest politics of the kind we’ve seen since Ferguson, and a wider effort to police the culture for hidden forms of racism, which don’t require tax increases to root out.

Finally, the late-Obama left is shaped by the success of the same-sex marriage movement, a rare example of a progressive cause that seems to be carrying all before it. To activists, its progress offers a model for winning even when electoral obstacles loom large: It shows that the left can gain ground at the elite level and then watch the results trickle down, that victories on college campuses can presage wider cultural success and that pathologizing critics as bigoted and phobic can be an effective way to finish up debates.

I suspect that a lot of the ambition (or aggression, depending on your point of view) from the campus left right now reflects the experience of watching the same-sex marriage debate play out. Whether on issues, like transgender rights, that extend from gay rights, or on older debates over rape and chauvinism, there’s a renewed sense that what happens in relatively cloistered environments can have wide ripples, and that taking firm control of a cultural narrative can matter much more than anything that goes on in Washington.

What’s interesting about this ambition is that it’s about to intersect with a political campaign in which the champion of liberalism will be a Clinton . when the original Clintonism, in its Sister Souljah-ing, Defense of Marriage Act-signing triangulation on social issues, is a big part of what the new cultural left wants to permanently leave behind.

Precisely because this left’s energy is cultural rather than economic, this tension is unlikely to spur the kind of populist, Elizabeth Warrenesque challenge to Hillary that pundits keep expecting.

But it does promise an interesting subtheme for the campaign. Can Hillary, the young feminist turned cautious establishmentarian, harness the energy of the young and restless left? Or will the excesses associated with that energy end up dividing her coalition, as it has divided liberal journalists of late?

Those of us watching from the right . with, perhaps, a little popcorn . will be interested to find out.