Maine Gov. Janet Mills addresses the Climate Action Summit in the United Nations General Assembly at U.N. headquarters, Monday, Sept. 23, 2019. Credit: Jason DeCrow / AP

BOSTON — The governors of three New England states and the mayor of Washington, D.C., signed a regional pact aimed at dramatically reducing transportation pollution while leaving time for states skeptical of the effort — including Maine — to sign on as well.

The Transportation and Climate Initiative Program is designed to reduce motor vehicle emissions by at least 26 percent by 2032. The initiative was signed Monday by Republican Massachusetts Governor Charlie Baker as well as Connecticut Governor Ned Lamont, Rhode Island Governor Gina Raimondo and Washington, D.C., Mayor Muriel Bowser — all Democrats.

The states account for about 73 percent of the transportation emissions and 76 percent of the vehicles in New England. The governors of other New England states — Maine, New Hampshire and Vermont — have expressed skepticism about the program, in part over fears that it amounts to a gas tax or a tax on carbon that would mostly burden people in rural areas.

The initiative will require large gasoline and diesel fuel suppliers to purchase “allowances” for the pollution caused by the use of the fuels they sell in the region. The allowances would decline each year, generating billions for states to invest in carbon-reducing transportation options.

In New England, transportation is responsible for over 40 percent of greenhouse gas emissions, while that figure in Maine is close to 50 percent. Exposure to air pollution can worsen lung and heart ailments, cause asthma attacks and increase the risk of other serious health conditions.

The regional effort has existed since 2010, but it was largely dormant until 2015 before picking up over the last three years. Maine has been participating in discussions around the initiative since just after Gov. Janet Mills took office in 2019, but her administration has signaled skepticism all along.

In a Monday interview, Mills said that one of her main hesitations on the initiative was what she saw as a lack of specificity, saying it was still a “theory.” She cited “continuing concerns” about potential effects on the price of gas, an issue she said would burden low-income Mainers.

“We have the most dispersed population of any state in the country,” she said.

Massachusetts has a goal of net-zero emissions by 2050, while Maine has separate goals of becoming carbon-neutral by 2045 and reducing emissions by 80 percent by 2050. The latter goal is more ambitious, since research led by the University of Maine based on 2007 and 2016 data found forests may already offset up to 75 percent of the state’s emissions.

States participating in the initiative have also agreed to invest 35 percent of annual revenue from the program in communities underserved by current transportation options and with disproportionately high levels of pollution. The initiative could generate more than $1.8 billion in Massachusetts by 2032, Baker added.

“By partnering with our neighbor states with which we share tightly connected economies and transportation systems, we can make a more significant impact on climate change while creating jobs and growing the economy,” Baker said in a statement.

Opponents have raised concerns about a potential gas price hike. If fuel companies pass the cost of the allowances onto consumers, the price of gas in the region could climb by five cents to 17 cents per gallon when the pact takes effect.

The deal will put even more pressure on businesses already struggling to survive during the pandemic, said Christopher Carlozzi, the Massachusetts director for the National Federation of Independent Business.

“The same small businesses that have faced shutdowns, countless restrictions, new regulations, and capacity limits will now face higher fuel costs due to Massachusetts joining the TCI,” Carlozzi said in a statement Monday.

Other states still have the ability to formally sign onto the initiative. The program is slated to begin in 2023 after a reporting year to allow for additional jurisdictions to sign on. Those states, in addition to the other New England states, include Delaware, Maryland, New Jersey, New York, North Carolina, Pennsylvania and Virginia.

Story by Steve LeBlanc, The Associated Press. Bangor Daily News writer Michael Shepherd contributed to this report.