Sen. Paul is right that Republicans have behaved hypocritically on fiscal matters, decrying spending and deficits only when Democrats control government. They have refused to make the sort of draconian spending cuts necessary to balance the budget while also adhering to their new orthodoxy of cutting taxes. But even Paul himself is guilty of this. For all of his rhetoric, he nevertheless voted for Republicans’ $1.5 trillion tax cut in December.

But this is not a betrayal of Republican principles, at least not the principles that have driven Republicans over the last four decades. The battle for the fiscal soul of the Republican Party was waged in the 1970s and 1980s. When most Republicans abandoned President Bush in 1990, it was clear that Republicans cared primarily about cutting taxes, not deficits.

Modern conservatism is built around the promise of supply-side economics — an economic philosophy that claimed to allow Republicans to cut taxes, maintain popular social programs, robustly fund defense and balance the budget. The problem: Supply-side economics hasn’t actually worked as promised, but the GOP refuses to abandon it.