BOSTON — When you can't win at the ballot box, sue the ballot box.

That's the strategy a coalition of union-backed progressives is now pursuing at Massachusetts' highest court, where lawyers spent Monday arguing that voters should not get to vote on a measure to cut the state income tax from 5% to 4%.

The complaint targets the summary written by Attorney General Andrea Campbell's office, claiming it's constitutionally inadequate because it doesn't tell voters the measure also reduces the long-term capital gains rate. If the Supreme Judicial Court agrees, the question disappears from the November ballot before a single voter weighs in.

Lew Finfer, director of Massachusetts Action for Justice, is among the plaintiffs. The court typically rules within 130 days.

According to a Tufts University analysis, the median Massachusetts household would save roughly $1,250 a year if voters approve the cut.

Opponents — including Governor Maura Healey — warn the cut would force reductions to MassHealth and other social programs.

The state estimates that, fully phased in by 2030, the cut could reduce annual revenue by roughly $5 billion.

The same warnings showed up the last time Massachusetts voters cut the income tax — and they didn't pan out.

In 2000, voters approved a phased rollback of the income tax from 5.95% to 5%. Opponents at the time warned the cut would "slash" $2.7 billion in revenue over four years. According to a Pioneer Institute analysis released this year, the actual first-year impact was about $530 million — roughly one-fifth of what critics had predicted. Inflation-adjusted income tax collections at the lower 5% rate ended up exceeding what the state had collected at 5.95%.

Pioneer Institute Executive Director Jim Stergios estimates the first-year impact of dropping the rate from 5% to 4% would be roughly $700 million — about 1.1% of the state budget — not $5 billion.

"We're now at rock bottom in terms of new business formation over the past couple of years," Stergios told WBUR. "On top of that, we've lost 35,000 jobs since 2020."

Massachusetts has been losing residents — and their tax dollars — for years.

IRS migration data show the state's net loss of adjusted gross income to other states grew from roughly $900 million in 2012 to $4.18 billion in 2023, according to the Pioneer Institute. Florida and New Hampshire alone accounted for about $2.75 billion of that loss. Florida pulled in more than 21,000 Massachusetts residents in 2023; New Hampshire pulled in more than 20,000.

Roughly 70% of the AGI Massachusetts lost in 2023 came from taxpayers earning $200,000 or more — the same bracket where the 4% millionaires' surtax voters added in 2022 also bites at the top end.

The people leaving for Florida and New Hampshire are voting with their feet. The plaintiffs at the Supreme Judicial Court are trying to make sure everyone else doesn't get a chance to do the same at the ballot box.