Two years ago Kansas embarked on a remarkable fiscal experiment: It sharply slashed income taxes without any clear idea of what would replace the lost revenue. Sam Brownback, the governor, proposed the legislation — in percentage terms, the largest tax cut in one year any state has ever enacted — in close consultation with the economist Arthur Laffer. And Mr. Brownback predicted that the cuts would jump-start an economic boom — “Look out, Texas,” he proclaimed.

But Kansas isn’t booming — in fact, its economy is lagging both neighboring states and America as a whole. Meanwhile, the state’s budget has plunged deep into deficit, provoking a Moody’s downgrade of its debt.

There’s an important lesson here — but it’s not what you think. Yes, the Kansas debacle shows that tax cuts don’t have magical powers, but we already knew that. The real lesson from Kansas is the enduring power of bad ideas, as long as those ideas serve the interests of the right people.

Why, after all, should anyone believe at this late date in supply-side economics, which claims that tax cuts boost the economy so much that they largely if not entirely pay for themselves? The doctrine crashed and burned two decades ago, when just about everyone on the right — after claiming, speciously, that the economy’s performance under Ronald Reagan validated their doctrine — went on to predict that Bill Clinton’s tax hike on the wealthy would cause a recession if not an outright depression. What actually happened was a spectacular economic expansion.