Sometime this spring, the Massachusetts Supreme Judicial Court will decide whether the so-called Fair Share Amendment can appear on the state ballot in November. The proposal would amend the Commonwealth’s constitution, which for more than 100 years has decreed that income may be taxed only at a uniform rate. If approved, the initiative would subject anyone with more than $1 million in income to a surtax 4 percentage points higher than the state’s regular income tax rate of 5.1 percent. The tax on million-dollar-plus incomes would thus rise to 9.1 percent — a prodigious 80 percent increase in the marginal tax rate.

Left-wing advocates have long resented the flat-rate income tax. They have repeatedly tried to get voters to approve an amendment to the Massachusetts Constitution doing away with it. Five times — in 1962, 1968, 1972, 1976, and 1994 — they have sponsored ballot initiatives to open the door to graduated tax rates. Five times voters have said no.

The latest effort, spearheaded by a coalition of labor unions and progressive activists called Raise Up Massachusetts, comes with two twists.

One is a lament about inequality. The coalition decries the fact that the richest Massachusetts households pay a smaller share of their income in state and local taxes than other households do. While Bay State tax filers in the top 1 percent expend less than 5 percent of their income on state and local taxes, those in the bottom quintile pay more than 10 percent. In the face of such an unfair and regressive tax system, argue the amendment’s drafters, boosting the income tax rate paid by the very wealthy is a matter of simple fairness.

The other twist is an assurance that Beacon Hill won’t have carte blanche when it comes to spending the annual $1.9 billion the new surtax is optimistically projected to raise. The legislature will instead be restricted from using the funds for anything other than what Raise Up Massachusetts calls “much-needed investments in transportation and public education.”

Neither claim holds up under scrutiny.

For starters, the Massachusetts income tax is anything but regressive. Households making more than $1 million collect 19 percent of all the income in Massachusetts — but they fork over 29 percent of all the income tax paid to the state. Households making $75,000 or less, by contrast, account for 23 percent of all income earned in Massachusetts, yet they generate a mere 7 percent of the state’s income tax revenue. By any yardstick, that’s a highly progressive arrangement.

Without violating the constitutional requirement of a uniform tax rate, the legislature has fashioned an income tax burden that falls much more steeply on the wealthy. Thanks to various deductions, exemptions, and credits, taxpayers in the lowest quintile pay an effective income tax rate of only 0.8 percent. Those in the highest quintile pay an effective rate of 4.2 percent — more than five times greater.

The regressive tax system that critics bewail is a function entirely of sales and property levies. If liberal activists want to lighten a tax bite that disproportionately affects those at the bottom of the economic ladder, those are the taxes they should be targeting. But skewing an already progressive income tax system even more sharply against the wealthy is the very opposite of a “Fair Share” reform.

Equally misleading is the suggestion that revenue from a millionaires’ tax will mean more funding for education and transportation. True, the proposed amendment declares that the money “shall be expended” only for those purposes. But Beacon Hill already expends far more on education and transportation than the $1.9 billion the surtax will supposedly raise. The MBTA budget alone is about $2 billion. In the most recent state budget, $7.1 billion goes for education.

Which means that if the amendment passes, the legislature will be under no obligation to change anything. The measure doesn’t say that spending on education and transportation must be increased. It doesn’t say that spending may not be decreased. It says only that revenue from the surtax must not be used for any other purpose. Since dollars are fungible, lawmakers will effortlessly be able to meet that obligation by affirming that $1.9 billion of the education or transportation budget had been funded through the new tax.

The Fair Share amendment may be well intended, but it combines bad policy with bad constitutional drafting. If the Supreme Judicial Court doesn’t spike this initiative, the voters certainly should.

Jeff Jacoby is a columnist for The Boston Globe.