Millionaires tax proceeds are supposed to bolster education and transportation. Lawmakers would decide if they actually do.

Legislative leaders offered few assurances about where they, let alone future legislators, would funnel the proceeds from the so-called millionaires tax.Carlin Stiehl for The Boston Globe

One television advertisement touts a proposed surtax on wealthy residents as a cure for the state’s “teacher shortage.” The initiative would mean better roads, another spot says. Nearly every single ad from proponents says it would raise $2 billion a year.

It ultimately may. But should voters embrace Question 1 and create a new 4 percent surtax on annual earnings above $1 million, the decision about where the money ultimately goes — and who benefits — would rest with lawmakers on Beacon Hill, who face no obligation to use the revenue exactly as proponents are pitching.

In interviews and statements, legislative leaders offered few assurances about where they, let alone future legislators, would funnel the proceeds from the so-called millionaires tax, often citing a laundry list of needs or competing interests.

The proposed constitutional amendment would require that the revenue it raises “only” go toward public education and transportation. But it also notes that the money would be “subject to appropriation,” or in other words, up to lawmakers. That, critics contend, leaves no guarantee spending on schools or roads or the MBTA would actually increase because lawmakers could simply shift other, existing revenue elsewhere.

Representative Aaron Michlewitz, the House’s budget chairman, said that if the initiative passes, lawmakers “should be directing money to those items” to fulfill the will of the voters.

“We’ll certainly try to uphold this end of the bargain. But you can never guarantee anything,” said the North End Democrat, who voted to put the question before voters. “You don’t know what the economic outlook will be. It would be disingenuous to say with a 100 percent guarantee [how it would be spent], with the idea that we may have to make decisions to offset other issues.”

The amendment includes some definition. It would push the revenue raised toward “repair and maintenance” of roads, bridges, and public transportation, which could be read as a prohibition on spending on expansion projects. The revenue could also go toward “affordable public colleges and universities,” indicating that its intent is to help keep the cost of higher education down, and for “quality public education.”

Still, even in earmarking the revenue for those pockets of education and transportation, the proposal allows for a wide universe of possibilities in categories that already account for billions in annual spending.

Supporters say that could mean dedicating millions of more dollars to the MBTA as it implements dozens of actions required following a federal probe. Hundreds of bridges around the state are in disrepair, they point out. Legislators repeatedly field calls to help cut the cost of college in Massachusetts or make early education more affordable.

But those are options, not commitments. Legislative leaders also declined to say whether they believe the revenue — pitched as helping both education and transportation — should be evenly split, saying the needs from year to year, as well as the actual amount the surtax raises, could dictate the division.

“Let’s get it across the finish line and then see what we actually have,” said Representative James O’Day, a West Boylston Democrat and a lead sponsor of the effort to get the question on the ballot. “We’ll figure out what the proportions are going to be once we sort of see what that number is.”

Leadership of the Fair Share committee includes an array of groups, including the Massachusetts Teachers Association, SEIU 1199, and the Coalition for Social Justice, among others. But the majority of the $18.8 million it had reported raising has come from teachers unions, with the MTA alone giving more than $11 million as of Tuesday morning.

The labor group deferred comment to the Fair Share committee, whose leadership has not discussed how the money should be split, said spokesman Steve Crawford.

“We have never had a discussion about individual priorities,” Crawford said.

Spokespeople for House Speaker Ronald Mariano and Senate President Karen E. Spilka said neither Democrat was available for interviews in recent days, and each issued statements supporting the initiative but offering no specific promises for where the money would go. Spilka, an Ashland Democrat, said senators would “turn to the public to listen, learn, and gather feedback” before deciding what education and transportation initiatives would benefit.

“These dollars have to be spent on education and transportation — and will all be additional dollars over and above funding that is already provided,” said Senator Jason M. Lewis, a Winchester Democrat and a leading sponsor of the measure. “That’s certainly what I will be advocating for.”

Critics paint a different, and more nefarious, picture. The Legislature, they argue, could simply replace the revenue it would have otherwise spent on education and transportation with the money raised by the new tax on high earners, and then push the equivalent elsewhere — all while keeping those spending buckets level.

It’s a point opponents made before the Supreme Judicial Court in unsuccessfully pushing to rewrite how the ballot measure would be summarized for voters.

The committee opposing the measure, funded predominantly by donations from Massachusetts business leaders, has leaned into this argument, casting the question as a politician-led effort to give themselves a “blank check with no accountability.”

“I don’t trust the politicians with my money,” Ann Sullivan, who owns Metro Equipment Corp. in Braintree, says in one ad.

Jim Stergios — executive director of the Pioneer Institute, a right-leaning think tank — likened the proposal to a shell game, with little ability for the public to track where the money goes particularly long into the future.

“A Legislature in 2034 doesn’t really care what the intentions of a Legislature in 2022 are,” he said.

Democratic legislators reject that argument as a red herring, saying the needs of the state’s schools and transportation system have, and will, outstrip resources. “I don’t worry about future legislators ignoring education and transportation,” said Senator Will Brownsberger, a Belmont Democrat.

Still, exactly how much money the amendment would raise is a matter of debate. The left-leaning Massachusetts Budget & Policy Center estimates the amendment is likely to generate at least $2 billion annually, but the Executive Office for Administration and Finance — the budget office under Governor Charlie Baker, who has said he personally opposes the tax hike — is less bullish, saying it may generate $1.2 billion “in the near term.” It also said that any revenue “will vary significantly and unpredictably from year to year.”

Tufts’s Center for State Policy Analysis estimates the state would see $1.3 billion in revenue in 2023, but its projection comes with nuance: The state could see $1.8 billion in direct receipts from the amendment, but it will also have to contend for an estimated $500 million loss because of “tax avoidance” or some wealthy taxpayers leaving the state entirely.

A “significant amount” of the new revenue could end up leaking into other areas, according to one brief from the Tufts center, and tracking the revenue — and holding legislators to account — is only made more difficult by the wide breadth of options for where the money could be spent.

“There is this kind of trust question,” said Evan Horowitz, the center’s executive director, “not just in people you vote for today but people you envision [voting for] in the future.”

Legislators also routinely dismissed Republican-led efforts to affix limits on legislative maneuvering in the proposed amendment. Representative Bradley H. Jones, the House minority leader, pushed an amendment in 2019 that sought to ensure any money raised would be in addition to money already being spent. (It was rejected.)

The same day, Senator Bruce E. Tarr, the Senate’s minority leader, proposed creating a new fund where the revenues would automatically be deposited, making it easier, he argued, to track how they’re spent. That, too, was defeated.

“Because it’s subject to appropriation, there’s no guardrail. There’s no standard,” said Jones, a North Reading Republican. “I’ve been around here long enough that I’m sure a lot of private promises have been made. ‘You’ll definitely get some of it. You’ll definitely get some of it.’ And there’s nothing like making $10 billion in promises for $1 billion [in revenue].”