'In my view neither major political party has libertarian inclinations. ... I'm slightly more sympathetic to the progressives who insist that I should really be a Democrat. They tell me "After all, you are rational. You believe in evolution and support carbon taxes and redistribution and think money was too tight during the Great Recession. You are pro-immigration and skeptical of the idea that America is an 'exceptional' nation, which must police the world." Those are all good arguments, but then I start obsessing about economics. After all, I am an economist.'
Apart from completely missing that the Democrat Party is the party of social freedom and the Republican Party is the party of economic freedom, it's rather singular for a self-described libertarian to embrace economic redistributionism so openly (not to mention a draconian form of taxation). To do so betrays a feeling for the left, not the right, which, if libertarians were only honest enough to admit it, has always been their inclination.
Sumner might reflect on the fact that we actually live in a perfect storm of libertarianism, in which economic (and social) actors have been unleashed to be all that they can be. The trouble is, only a few "succeed". The fact that income inequality has reasserted itself to a degree not seen since the gilded age is proof of the basic fact that not all men are created equal. The very best at making money have risen to the top and become enormously wealthy in an environment specifically designed to allow it to happen. The end result of economic libertarianism is that the very best will eventually succeed in hoarding all the goodies for themselves while the rest of us are left to serfdom. The end result of libertarianism is freedom for thee, but not for me.
The same can be illustrated on the social side, where some freak flags fly higher than all the rest. They rise to fame and influence beyond all their fellows in "art", "music", "literature" and "society", if you can call violent, vulgar and obscene Hollywood films, rap, "shady" novels and the Kardashians representative of those categories.
Conservatism, primarily rooted in religion, has historically functioned in society to apply the brakes to keep these actors from getting out of control and acquiring undue influence, whether socially or economically. The left only imagines itself capable of replacing religion's heretofore tempering role, which primarily functioned through willful self-restraint. Hence the efforts to reduce income inequality by force through taxation schemes, which obviously aren't working. On the social side the left has had even less success, except by recourse to venomous speech and conduct codes which meet with little assent and not a little fear and loathing among the many.
Freedom, as currently conceived in all its sterility, is quite literally killing America.