Last month, Paul Krugman launched a campaign to pin the financial crisis on “Reagan and his circle of advisers.” It was “Reagan-era deregulation,” Krugman wrote, that led us to the “mess we’re in.” The substance of Krugman’s screed was so tissue-thin that even ultra-liberal columnist Robert Scheer responded to it with bafflement: “How could Paul Krugman, winner of the Nobel Prize in economics and author of generally excellent columns in the New York Times, get it so wrong?” So let’s not credit Krugman with making a serious argument, rather than throwing a partisan grenade.

The broader narrative into which Krugman’s column …