Mr. Mulligan’s empirical research puts the best estimate of the contraction at 3%. The CBO still has some of the economics wrong, he said in a phone interview Thursday, “but, boy, it’s a lot better to be off by a factor of two than a factor of six.”

The CBO’s intellectual conversion is all the more notable for accepting Mr. Mulligan’s premise, which is that what economists call “implicit marginal tax rates” in ObamaCare make work less financially valuable for lower-income Americans. Because the insurance subsidies are tied to income and phase out as cash wages rise, some people will have the incentive to remain poorer in order to continue capturing higher benefits. Another way of putting it is that taking away benefits has the same effect as a direct tax, so lower-income workers are discouraged from climbing the income ladder by working harder, logging extra hours, taking a promotion or investing in their future earnings through job training or education.

The CBO works in mysterious ways, but its commentary and a footnote suggest that two National Bureau of Economic Research papers Mr. Mulligan published last August were “roughly” the most important drivers of this revision to its model. In short, the CBO has pulled this economist’s arguments and analysis from the fringes to center of the health-care debate.