The party needs to turn away from what Louisiana Gov. Bobby Jindal calls “dumbed-down conservatism.”

“We need to stop being simplistic ... and we need to stop insulting the intelligence of the voters,” Jindal told Politico this week.

He’s spot on. Taking conservatism up-market intellectually will mean weaning the party off the seductive anti-Obama mythomania of Fox News; off the pugnacious partisanship that personally profits commentators such as Rush Limbaugh, Laura Ingraham and Ann Coulter; and off the cross-eyed conspiracy theories of professional buffoons such as Donald Trump.

But more importantly, it will require a return to politics and policies based on fact and evidence. And perhaps nowhere is that need more acute than on economics.

Since the election of Ronald Reagan, supply-side economics — in brief, the belief that lower tax rates, particularly on upper earners, will increase incentives to produce things, resulting in economic growth that benefits everyone — has been the doctrine of the Republican Party. The Wall Street Journal editorial page has become a reflexive cheerleading squad for supply-sidism, while powerful groups such as the Club for Growth and Americans for Tax Reform have policed party ranks for deviations from the approved theology.

The adherence to supply-side theory has come despite, rather than because of, real-world results. Much of the Reagan-era prosperity attributed thereto was really the result of Keynesianism that dared not speak its name. That category includes the big, deficit-financed defense build-up and income tax cuts that boosted consumer spending.

Meanwhile, the past three decades have proved key supply-side tenets crashingly wrong. Chief among them is the assertion that lower marginal income tax rates for upper earners are an essential catalyst to growth. Another is the related warning that tax increases on the well-to-do will inevitably cause economic calamity.