The most momentous political news of the past week? For my money, it wasn’t James Comey’s Senate testimony, riveting as it was. It was the Kansas Legislature’s decision to defy the governor and raise income taxes — a move that could well be the first step in a transformation of American politics much more far-reaching than anything that could come from Russiagate.

Hear me out. Kansas, under Gov. Sam Brownback, has come as close as we’ve ever gotten in the United States to conducting a perfect experiment in supply-side economics. The conservative governor, working with a conservative State Legislature, in the home state of the conservative Koch brothers, took office in 2011 vowing sharp cuts in taxes and state spending, except for education — and promising that those policies would unleash boundless growth.

The taxes were cut, and by a lot. The cumulative cut was forecast to be $3.9 billion by 2019. A fellow at a right-leaning Missouri think tank said in 2015 that Mr. Brownback’s cuts were “the biggest tax cut of any state, relative to the size of its economy, in recent history.”

The cuts came. But the growth never did. As the rest of the country was growing at rates of just above 2 percent, Kansas grew at considerably slower rates, finally hitting just 0.2 percent in 2016. Revenues crashed. Spending was slashed, even on education: In March, the State Supreme Court ruled that state-level school spending was unconstitutionally low. The court is ideologically mixed, but its ruling was unanimous.