About half of the $42.7 billion Senate budget proposal is consumed by spending in just three areas — health care, pensions and debt service — leaving some senators worried about underinvestment in other priority areas like transportation, housing and the environment.

Spending on MassHealth, the health care program that serves 1.8 million seniors, children and low-income residents, totals $16.55 billion, or about 38 percent of the budget, Senate Ways and Means Committee Chairman Michael Rodrigues said during remarks on the first day of fiscal 2020 budget deliberations.

Under questioning from Senate Minority Leader Bruce Tarr, Rodrigues said proposed spending on public employee pensions, at $2.84 billion, accounts for 6 percent of total spending. And spending on debt service is set to rise by $86 million to $2.56 billion, also accounting for 6 percent of state spending.

Despite channeling so much money into just a few areas, Rodrigues said the Senate budget allocates $5.176 billion to Chapter 70 aid to local school districts, an increase of $268 million over this fiscal year and the largest increase in state spending in that account in two decades.

While Rodrigues touted other new investments, such as plans to shore up special education spending and deliver aid to struggling nursing homes, other senators said tax hikes deserved consideration, beyond the $14 million tax on opioid manufacturers and the $12 million tax on vaping products included in the Senate budget.

According to Sen. Jamie Eldridge, senators had offered amendments that would raise another $600 million in revenue, though much of those proposals will not be subject to debate this week. One amendment was withdrawn immediately by its sponsor as a handful of senators made their case for what Eldridge described as "progressive taxation," or "raising taxes on large corporations and raising taxes on the wealthy."

An Acton Democrat who chairs the Senate Progressive Caucus, Eldridge said taxes were cut by approximately $3 billion in the late 1990s and into the early 2000s, leading to situations like a "multi-year backlog" on approving permits at the Department of Environmental Protection, one Department of Conservation and Recreation maintenance crew to serve the entire state, and thousands fewer families receiving rental assistance vouchers than in 1993 despite growth in the state's population since then.

Sen. Becca Rausch filed and withdrew an amendment that sought to raise the corporate tax rate from its current 8 percent to 9.5 percent, which she said would yield an estimated $375 million for the state.

"What would you put on your $375 million wish list?" the Needham Democrat asked her colleagues, ticking off possibilities like slashing public transit rates in half, restoring MassHealth dental benefits, funding full-day kindergarten statewide, expanding broadband service and providing home care workers to every elderly person who needs one.

Rausch said she withdrew her amendment "in deference to this body's commitment to work on revenue this session," wishing luck to Pittsfield Sen. Adam Hinds as he leads a Senate working group that's expected to develop tax policy recommendations for the 2021-2022 legislative session.

Hinds, the Senate chair of the Revenue Committee, described the effort as a "robust process in place to take a comprehensive look at our system of revenue."

"It is my hope that before we take up large proposals during this budget process that we allow the current working group to complete its work," he said.

The House chair of the Revenue Committee, Rep. Mark Cusack, made a similar ask of his colleagues when the House debated its spending bill last month, telling his colleagues the legislative panel needed time to review the bills that had been filed. Representatives withdrew the revenue amendments they had filed, without debate.

Eldridge later on Monday withdrew his two amendments that remove a sales tax exemption for aircraft and close what he describes as a loophole involving the single sales factor for mutual fund corporations.

"I would say that right now we don't have enough support to pass the measures, but today is really an opportunity -- and really the whole week -- to raise the fact that most of the amendments we will file won't pass this week, because there's not enough revenue to fund the important programs and services that we're all fighting for," Eldridge said.

Still pending before the Senate as of Monday afternoon was a proposal from Sen. Jo Comerford that would reform the corporate minimum tax. Comerford, a Northampton Democrat, said the minimum tax has been static for 30 years, and that implementing a tiered structure would generate between $40 million and $50 million in revenue she proposed using to fund school transportation.

Comeford said she was "heartened" by the revenue working group and the "important tax increases" in the budget but also "concerned about the dearth of revenue and about the impact that has on the spending that we all want to do for our communities, for our constituents."

The Senate budget calls for a 3.1 percent increase in total state spending, or $1.27 billion above the budget approved for fiscal 2019, but spending in the budget bill will rise only $840 million above projected state spending once midyear spending and continued appropriations are factored into the equation, Rodrigues said.

While some senators said the state is failing to make adequate investments in programs and services, others touted steady deposits into the state's rainy day fund. Rodrigues said that between scheduled deposits and possible fiscal 2019 surplus revenues, the stabilization fund balance could soon exceed $3 billion.

But concerned with trying to soften impacts on services when the next recession hits, Sen. Viriato deMacedo said the state, given its level of spending, should be eyeing a $4.3 billion rainy day fund balance. The fund, he said, had close to $2.6 billion in it in 2008, when the state budget totaled about $27 billion.

Uncertainty over federal funds, which play a major role in the state budget, also continues given the climate in Washington where partisan disagreements abound over priorities, and even keeping the government running.

Rodrigues said the state budget incorporated about $11.5 billion in federal aid this year and next year's budget plan relies on about $11.65 billion in federal funds, which he characterized as "level funding."