New York Gov. Andrew Cuomo was on the road to Albany when a light flashed before him. Lo, said a voice from the light, progressive taxes are driving out high earners and damaging the state budget.

Some miracle like that must have happened because on Monday Mr. Cuomo awakened from his first eight years as Governor and according to the Buffalo News declared, “Tax the rich, tax the rich, tax the rich. We did that. God forbid the rich leave.”

Mr. Cuomo delivered this testimony from the Book of Tax Revelation while announcing Monday that New York state’s income tax revenue over the last two months was $2.3 billion below projections. “That’s as serious as a heart attack,” he said.

Mr. Cuomo blamed in particular the 2017 federal tax reform, which limited the deductibility of state and local taxes to $10,000. “It literally restructured the economy to help red states at the cost of blue states,” he groused. “It was a diabolical, political maneuver.”

He would know about tax sadism. New York City’s combined state and local top rate of 12.7% hits taxpayers earning more than $1 million and is the second highest in the country after California. The deduction limit raised New York’s top rate by an effective 5%, though this was partially offset by the tax reform’s 2.6 percentage-point reduction in the federal top rate.

The reality is that rates on wealthy New Yorkers rose more during the Obama years due to the 3.8% Medicare surcharge on investment income and expiration of the Bush tax cuts for high earners. But Mr. Cuomo is right that tax reform has made New York less economically competitive by fully exposing high earners to its progressive taxes.

The top 1% of New York taxpayers pay 46% of state income taxes. Revenues vacillate with capital gains—a problem that is compounded in New York because bonuses in the finance industry are often tied to trading revenue. Markets were especially volatile during the last quarter amid uncertainty about trade and interest rates.

Some New York taxpayers moved income forward to 2017 to take advantage of the uncapped state-and-local deduction, which boosted revenue last year. Politicians naturally spent that windfall. But non-wage income-tax revenue over the last two months were still $1.2 billion below 2016 levels.

The bigger problem seems to be geographic tax arbitrage. Mr. Cuomo notes in a PowerPoint presentation that “anecdotal evidence suggests that high income taxpayers are considering changing their residence and that financial industry firms are looking at real estate outside of New York.” While the Governor blames the GOP tax reform, high earners have been decamping for years, as E.J. McMahon of the Empire Center has chronicled.

According to IRS data we’ve examined, New York state lost $8.4 billion in income to other states in 2016 (the latest available data), up from $4.6 billion annually on average during the prior four years. Florida raked in the most New York wealth. Mr. Cuomo says that “a taxpayer in Florida would see no increase, or a decrease” under the GOP tax reform and “Florida also has no estate tax.” New York’s 16% estate tax hits assets over $10.1 million.

During his 2010 campaign, Mr. Cuomo promised to let New York’s tax surcharge on millionaires expire. But he has extended it again and again and now wants to renew it through 2024 because he says the state needs the money. Meantime, he warns that a wealth exodus could force spending cuts for education and higher taxes on middle-income earners.

All of this was inevitable, as we and others warned. Yet rather than propose to make the state’s tax burden more competitive, Mr. Cuomo rages against a tax reform that has helped the overall U.S. economy, even in New York. Perhaps now that he’s found Art Laffer on the road to Albany, he’ll think anew.