Traffic is seen on Storrow Drive in Boston in this file photo. State House News Service photo

The controversial regional effort to reduce vehicle emissions and boost transit funding along the East Coast appears to have dwindled, at least for the time being, to include just Massachusetts, Rhode Island and Washington, D.C. as active participants.

The idea for the compact was launched last year with 13 states from Vermont to North Carolina signaling their support.

Connecticut Democrats and Gov. Ned Lamont, working under the pressure of a mid-week deadline to pass legislation this year and to finalize a new biennial state budget, scrapped the Nutmeg State’s implementation of the Transportation and Climate Initiative Program (TCI) as part of a compromise reported Friday by the CT Mirror.

Though Connecticut will not implement TCI in its next budget, the CT Mirror reported Monday that Democrats in Hartford “said the Transportation Climate Initiative might get a vote this year, or might be revisited in 2022.”

In Massachusetts, Gov. Charlie Baker has the authority to enter into a multi-state carbon reduction pact like TCI on his own, but governors in most other states need to secure legislative approval. Baker’s administration said Monday it remains committed to the program.

The three southern New England states and the nation’s capital city agreed in December to work towards implementing a regional “cap-and-invest” program to limit carbon pollution from cars and trucks and generate resources to expand clean transit options and improve public health. The coalition settled on a carbon emission reduction target of 26 percent by 2032, which could add an estimated 5 to 9 cents to the price of a gallon of gas, according to officials involved in the effort.

TCI is a central part of Baker’s transportation and climate agenda and is “critical” to the efforts that will be needed to achieve his administration’s goal of net-zero emissions by 2050, Energy and Environmental Affairs Secretary Kathleen Theoharides said in December when the three states and D.C. agreed to move forward with TCI.

On Monday, a Theoharides spokesman said the administration “understands the challenges” involved in developing a program like TCI, “but continues to believe the program’s capacity to combat climate change and build better, cleaner transportation infrastructure is unmatched.”

“The commonwealth has always planned to move forward with implementation only with multiple states participating. The program will continue to advance with the planned reporting year to gather more information about the program’s immense benefits for Massachusetts’ transportation system,” spokesman Craig Gilvarg said.

Before TCI can truly take effect, at least three jurisdictions must have authorized implementation of their respective programs. The memorandum of understanding signed in December calls for the TCI Program to begin as soon as Jan. 1, 2022 with a reporting year and for the first three-year compliance period to begin Jan. 1, 2023 “or at such later time as at least three jurisdictions have completed the legal processes required to implement their individual programs.”

Though Delaware, Maryland, New Jersey, New York, North Carolina, Pennsylvania, Vermont and Virginia all signed a statement of support for the concept of TCI and pledged to continue to partner with states that sign onto the compact, none have added themselves to the MOU since it was first announced in December.

The Massachusetts Fiscal Alliance, which has long been a vocal opponent of TCI, said Connecticut putting the implementation of TCI off for the year “is great news for taxpayers everywhere.”