Among his many talents, Ronald Reagan had a special gift for proving his critics wrong. Almost none of the leading economists of the late 1970s thought that his supply-side tax-cutting agenda, along with stable monetary policy and deregulation, could revive the economy from the malaise of that decade. But the prosperity of the 1980s — with growth rates higher than 6 percent — proved the Gipper correct. An economics major from Eureka College understood our free-market system better than the so-called experts at Harvard, Yale, and MIT.

As the 2016 presidential campaigns gear up, Reagan’s policies are under assault again, this …