The ‘millionaires tax’ was pitched as a $2 billion revenue source. It’s blown past that, yet again. - The Boston Globe

Billionaires tax is more like it.

The state’s surtax on its highest earners has already generated more than $3.1 billion in revenue this fiscal year, with still two months to be counted, likely leaving lawmakers with a generous surplus to dole out next spring.

The amount, disclosed in a letter released by the state Department of Revenue, already tops the $3 billion the state collected from the so-called millionaires tax during fiscal year 2025. It also far surpasses the $2.4 billion the state projected to spend from the levy in the fiscal year that ends next month.

The constitutional amendment approved by voters in 2022 applies a 4 percent surtax on annual income “in excess” of $1 million. But the measure also included a trigger linking that seven-figure threshold to any changes in the cost of living, meaning the amount someone has to earn to hit the tax increases with inflation. For tax year 2026, for example, only income over $1.1 million is now taxed.

Before the measure passed on the ballot, the Massachusetts Budget and Policy Center, a left-leaning think tank, projected it could generate at least $2 billion a year.

It’s repeatedly topped that. Around this time last fiscal year, the surtax had already produced $2.6 billion in revenue. The year before, it had produced about $1.8 billion by around this time.

The estimates immediately buoyed supporters’ claims that the surtax would deliver much-needed revenue for the state despite fears it could drive some of the state’s wealthiest residents to move to locales with lower tax burdens.

Federal data show that people are, in fact, leaving the state. IRS data released in March found that nearly 30,000 more people left Massachusetts than came into it from 2022 to 2023, according to tax returns — one of the highest numbers of any state in the country.

Within the highest-earning bracket — those earning $200,000 or more — 8,676 more individuals left the state than came into it from 2022 to 2023. That represents around 29 percent of the total net loss, data show.

Phineas Baxandall, director of research and policy analysis at the Massachusetts Budget and Policy Center, said while the surtax revenue has grown, so too has the Legislature’s reliance on it to fund tranches of the budget, including child-care grants or sending more money to the MBTA.

“While the money is growing, the knowledge of the vitality of this source is growing even faster,” he said, noting that state lawmakers originally budgeted just $1 billion in so-called millionaires tax money, and are now planning to commit $2.7 billion next fiscal year.

Andrew Farnitano, a spokesperson for Raise Up Massachusetts, the union-backed group that pushed the 2022 ballot initiative, argued the ever-increasing funding signals that “the ultra-rich are clearly staying in Massachusetts and paying more in taxes.”

This year’s haul should come as a welcome sign for lawmakers who are in the final stages of passing a budget for the next fiscal year during an uncertain economic forecast.

Amid mounting fears of a recession, the potential of deep cuts to federal aid, and uncertainty about where President Trump could steer federal economic policy, lawmakers are on edge. The Trump administration has already pulled back federal spending through cuts to school aid, health funding, and deep cuts to Medicaid.

“We know we are heading into strong headwinds,” Senate President Karen Spilka told reporters on Tuesday ahead of the first day of her chamber’s budget debate. “We are especially hard hit here in Massachusetts, losing billions of dollars. So we are fiscally responsible and future-focused.”

Evan Horowitz, executive director of Tufts University’s Center for State Policy Analysis, warned that lawmakers must still be “defensive” with millionaires tax money, even in the face of an economic downturn.

The surplus, he said, is fueled by business sales and equity returns, which are directly tied to the health of the stock market.

“The stock market has been booming, and so the millionaires tax is bringing in revenue,” he said. “This money is volatile. If the state builds a budget that relies on this money for core programs, you are going to have a hole in core programs [in the future].”

Samantha J. Gross can be reached at samantha.gross@globe.com. Follow her @samanthajgross.