Kevin Drum asks a good question.

Reading Tim Pawlenty’s paean to double plus supply-side-ism yesterday made me wonder, once again, why conservatives think we liberals are opposed to it. I mean, if it actually worked, why would we be? It’s politically popular, and by their accounts it would generate trillions of dollars in extra revenue that we could use to finance our beloved lefty social programs. What’s not to like?

The only answer I can come up with is that conservatives are now completely invested in their theory that we liberals loathe rich people so much that we don’t care. We all want to screw the wealthy so badly that we’re willing to forego the elections we’d win and the mountains of revenue we’d gain if we lowered their taxes. We hate them that much.

And Jon Chait explains that Kevin’s assumption is spot-on.

The early supply-siders, like Jude Wanniski and Jack Kemp, were sure they could convert liberals to their theory once they had learned the Good News. They actually viewed liberals as their most promising potential converts, precisely because they believed they had unlocked the key to higher revenue at no cost. They genuinely, tirelessly evangelized for years.

Eventually, pretty much all of them gave up on this hope. Now they almost all believe liberals hate the rich so much they’re willing to sacrifice economic growth and revenue in order to punish them.

But then there’s the other question: why conservatives continue to support trickle-down tax breaks for the rich, since they don’t work.

The left, after all, is incredibly pragmatic. If the right could demonstrate with incontrovertible proof that cutting taxes for the wealthy would generate more economic growth and greater government revenue, lefties would sign on. Liberals don’t much care if government is bigger or smaller; we care about end results — making a material difference in the lives of people. To that end, the left wants to do more of what works and less of what doesn’t.

But notice how the inverse never seems to apply. The left could demonstrate with incontrovertible proof that cutting taxes for the wealthy doesn’t expand the economy and can’t bring in additional revenue … and it wouldn’t make a bit of difference to the right. For conservatives and today’s GOP, the point isn’t to do what works; the point is to satisfy ideological goals. Evidence is interesting, but not determinative.

And the efficacy of economic agendas is nice, but conservatism’s larger philosophy matters so much more.

Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.