Gov. Charlie Baker is proposing a $40.5 billion state budget for next fiscal year that doesn't raise taxes but would change how the state goes after them and would start requiring some online-only, out-of-state companies to begin collecting sales tax, moves his administration say will mean millions more in tax revenue for the state.

The spending proposal, the full details of which will be released this afternoon, marks Baker's third since taking office in 2015, and funnels more money into agencies like the Department of Children and Families as well as the state's rainy day fund.

The Republican sticks to his pledge, repeated during last night's State of the State address, not to propose any broad-based tax increase. But his budget proposal recommends a number of "modernization" pitches his administration say will earn the state $187 million more in revenue.

Among them is a plan to require out-of-state, online retailers who do more than $500,000 in sales annually to begin collecting and remitting sales tax, if they don't already. Baker's budget office said it doesn't believe the change requires a new law, but instead will apply an existing definition of "economic activity" to companies who don't have a physical presence in Massachusetts but do "significant" online business.

Baker aides said the move isn't a new tax, but puts a focus on those major retailers who are not already collecting sales tax.

Also included is a previously announced plan to subject operators of short-term rentals, like Airbnb, who rent rooms or houses more than 150 days a year to the state's hotel tax. Baker aides estimate that could bring in $12 million.

Another $95 million would be collected by changing when the state collects already remitted sales taxes.

Baker aides said the total is the only one-time revenue they're relying on to balance the $40.5 billion spending plan, a roughly 90 percent drop from the $1.2 billion the state relied on in fiscal year 2015.

Spending overall increases 4.3 percent under the plan, which also puts an additional $27 million into the state's Department of Children and Families and $98 million into the state's rainy day fund.

The budget also includes price caps that are targeted at slowing state health care cost increases and a new "assessment" that would generate $300 million.

Spending on MassHealth makes up nearly 40 percent of spending of the entire budget and Baker aides are projecting a 4 percent growth in enrollment, which will push those enrolled to more than 2 million, or roughly 30 percent of the population.

Wrapping their arms around the rising cost has been a major focus of state officials, though uncertainly looms ahead amid promises from Congress and President-elect Donald Trump to dismantle the Affordable Care Act.

The MBTA, in addition to getting more than $1 billion in sales tax revenue, would also get another $187 million from the state to help balance its book, including $60 million in capital dollars. It's the same total amount it's received in each of Baker's budgets.

The proposal "keeps the Commonwealth of fiscal responsibility, essentially achieves our stated goal of bringing the state budget into structural balance, addresses long-term obligations and funds important priorities," Baker said in a letter sent to lawmakers.