Krugman reiterates a political point which should be reiterated -- and reiterated -- by liberal pols with the pseudoconservative right's magnificent artistry for that sort of messaging thing:
[T]he right has always seen Keynesian economics as a leftist doctrine, when it’s actually nothing of the sort.
Keynesian economics is merely modern macroeconomic theory in the same vein that evolutionary biology is modern biological theory. Because Keynesianism is now empirically grounded and thus subject to towering heaps of statistical verification, it's no more a falsified "theory," as the right would have it, than the Austrian School is cutting-edge analysis.
As Krugman notes, the right's motive behind characterizing Keynesianism as "leftist" is that the decency-averse right appreciates all too well the socioeconomic efficacy of increased government spending (job creation) during economic downturns. And, as Krugman delights in pointing out, the right itself uses Keynesian arguments (though it certainly doesn't label them as such) against potential defense-spending cuts and their consequent, losing effects on American jobs.
This basic concept -- Keynesianism -- is not economics graduate-seminar stuff, although God knows that professional economists have, in their uniquely disputatious way, managed to splinter it into a multifront, multischooled war of academic carnage. The elementary principle, however, remains intact: In times of economic slump, government spending creates jobs.
And liberal pols shouldn't be shy about aggressively barking the indisputable truth.