SINCE JANUARY, when two proposals to update the state’s 26-year-old formula for funding Massachusetts schools were filed, one by Gov. Charlie Baker and another by a group of lawmakers led by Sen. Sonia Chang-Diaz, it was clear that there was a big difference between the bills in how much new state money they allocated for districts across the state. A new report by a Boston think tank spells out details of just how widely they differ, saying the Chang-Diaz bill, dubbed the Promise Act by its sponsors, would steer nearly $1 billion more per year to schools by 2026 than Baker’s proposal.

The report, released Monday by the left-leaning Massachusetts Budget and Policy Center, does not take a formal position on the bills, but it is sure to be seized on by supporters of the Promise Act, including district leaders, education advocates, and teachers unions, who have said the state needs to make a much bigger investment in schools, particularly those educating lots of low-income students, than called for in the governor’s legislation.

The formula used to direct aid to districts considers a set of variables to determine how much money the state should contribute to local schools. The big difference between Baker’s bill and the Promise Act is in the increase they provide for educating low-income students. All told, the governor’s bill would direct an additional $460 million per year to districts beyond what they would get under the current formula, according to the report. The report says the Promise Act would steer an additional $1.4 billion to districts, or $946 million more than the governor’s bill.

The report projected the impact of the two bills on state funding for each district in the state, with the biggest differences emerging for Gateway Cities, the state’s former industrial hubs, which are home today to lots of lower-income families.

In Springfield, for example, the report says state aid would increase by $43 million under Baker’s bill and by $150 million under the Promise Act. For Brockton, the increases would be $25 million versus $70 million, while for Worcester, the increases would be $34 million versus $108 million. The state’s Gateway Cities have difficulty funding school costs with local property tax revenue, and the state pays the bulk of education costs in the communities.

“We know there are major gaps between what our Gateway Cities can invest in schools and how much our affluent cities can, and that’s connected to the outcomes we’re seeing in our schools,” said Colin Jones, a senior policy analyst at Mass. Budget and coauthor of the report. In trying to close disparities in student achievement, he said, “when a bill like the Promise Act goes as far as doubling the money for low-income kids, it’s going to make that a lot more possible compared to Gov. Baker’s bill, which adds 20 percent.”

A 2015 commission concluded that the state was underfunding schools by more than $1 billion because the aid formula had not kept pace with rising costs, particularly for employee health care, special education, English language learner services, and the added costs of educating low-income students.

Pressure has been steadily building since then to overhaul the funding formula. Lawmakers took a stab at reworking the formula last year, but House and Senate negotiators could not reach agreement before the session ended.

The sponsors of both bills now in play say their proposals fully honor the commission’s recommendations. The big difference in their funding numbers is because the 2015 report agreed only on a range within which the aid increase for low-income students should land, not a specific figure.

A spokeswoman for Education Secretary Jim Peyser underscored a point Baker made in January when he unveiled his bill. “The governor’s plan is funded with existing revenues, with investments beginning immediately and can be sustained over time,” she said in a statement.

Chang-Diaz, the lead sponsor of the Promise Act together with state Reps. Mary Keefe and Aaron Vega, acknowledged at the bill’s rollout that it would require new state revenue to be fully implemented. It’s not clear what the revenue source would be for the level of spending called for in the bill. Legislators have renewed an effort to enact a tax surcharge on income greater than $1 million, but it will be several years before such a measure, which requires a constitutional amendment, could be in place.

While much of the difference between the two bills is explained by the higher amounts of aid the Promise Act would earmark for Gateway Cities educating lots of low-income students, the bill would also be a boon for some wealthy communities because of a provision related to reimbursing districts for charter school students.

Boston and Cambridge, which have lots of charter school students but also a strong local tax base to support schools, would see big increases in aid under the Promise Act because of the charter reimbursement provision, which Boston Mayor Marty Walsh is strongly backing.

For Boston, the Promise Act would be a school funding windfall, with the city projected to get an additional $91 million a year in state aid by 2026 compared with virtually no increase ($8,000 a year) under Baker’s bill. Cambridge, whose rich tax base allows the city to fund schools at nearly twice the minimum level required by the state, would see $1.4 million a year in new state funding under the governor’s bill and $9.4 million under the Promise Act.

The Mass. Budget report projects that $121 million of the $1.4 billion in increased state aid from the Promise Act would go to the new charter reimbursement provision.

While there is broad agreement on the need for more education funding, particularly in districts that are the most financially strapped, Baker insists that any update of the school funding formula should come with new reforms aimed at driving improved student outcomes with that money. His bill contains several such provisions, including a fund for the state education commissioner to direct discretionary spending to promising district initiatives aimed at closing achievement gaps. Sponsors of the Promise Act, and the teachers unions backing it, have resisted talk of introducing new accountability measures, arguing that the Legislature should focus solely on boosting spending.

Baker’s bill and the Promise Act seem likely to define the funding range at play, with eventual increase in state funding likely to land somewhere between the figures called for in the two bills.

A third bill, sponsored by Rep. Paul Tucker of Salem, also proposes an update of the funding formula, but was not included in the Mass. Budget analysis because it leaves open exactly how much additional funding should go to districts for low-income students.

Sen. Jason Lewis, the Senate chair of the Education Committee has said he hopes to have a bill crafted by the end of June. The House chair of the committee, Rep. Alice Peisch, has been more circumspect in committing to a clear timetable, but has expressed her resolve to try to reach agreement on a bill.