Saying it would be a “terrible mistake” for the state to embark upon an expansion of South Station at the expense of a rail tunnel linking that hub with North Station, a coalition led by former Gov. Michael Dukakis is working to ramp up advocacy for the rail link in an effort to convince Gov. Charlie Baker of the project’s merits.

A letter originally penned by 21 lawmakers in 2012 to the Federal Railroad Administration in support of the North-South rail link now bears the signatures of more than 150 representatives and senators, including House Speaker Robert DeLeo. Proponents are hoping the critical mass of support and increased education efforts will be enough to sway the governor.

Dukakis, with the help of Sen. Jamie Eldridge of Acton and Rep. Sean Garballey of Arlington, convened a private meeting at the State House on Monday of the newly formed North-South Rail Link Working Group.

“The goal is to persuade the governor that South Station expansion makes no senses at all,” Dukakis said after the meeting, calling Baker a “thoughtful guy” who’s “obviously interested in doing what’s right.”

The group, according to the former governor, aims to engage in traditional means of advocacy and public education to build the case for a tunnel that they say would allow for uninterrupted rail service from Washington D.C. to Canada. With 34 members to start, including the mayors of Somerville, Salem, Quincy and Newton, the working group plans to meet monthly.

“I think it’s really a grassroots strategy speaking to the fact that there is support for this outside of Boston and also has benefits beyond the transportation benefits but economic development, housing benefits reducing traffic that reach, I would say arguably, all across Massachusetts,” Eldridge said.

Eldridge and Garballey co-chair the MBTA Caucus in the Legislature.

Noting that over 100 legislators have indicated support for the project, Eldridge said, “But I think, perhaps, they don’t understand the benefits to their districts about completing the North-South rail link so now it’s about a more comprehensive education to our colleagues as well as to people across the state as to the benefits of this project.”

The meeting came two weeks after Dukakis and former Gov. Bill Weld, another supporter of the North-South rail link, met with Baker to bring him up to speed on the project. The governors want Baker to release $2 million already approved by the Legislature for a study of the rail tunnel.

While Baker said he hasn’t ruled out supporting the rail link, he has expressed skepticism about its ability to reduce train congestion and said he does not see the project as mutually exclusive from an expansion of South Station.

Baker said the idea of expanding South Station interests him as both a way increase rail capacity into Boston and open up the Fort Point Channel area for development if a deal can be reached to relocate the U.S. Post Office building.

Dukakis reiterated his contention that a South Station expansion would be only a short-term fix for the rail system, and would need to be followed up soon after with an expansion of North Station.

“Expanding two stub-end stations is a 19th century solution to a 21st century problem,” he said.

Garballey told the News Service after the meeting that the number of lawmakers signing on to the 2012 letter, including 16 new members just this year, demonstrates “support for this project, certainly showing the desire to have an analysis done so that we can have solid figures to move forward on.”

The original letter referenced by Garballey was sent by lawmakers to the Federal Railroad Administration and requested that the rail link be a “key component” of the administration’s environmental impact statement for the Northeast Corridor of high-speed rail.

Project supporters continued gathering co-signers to the letter during the 2013-2014 session and this session. The list of co-signers includes Martin Walsh, who signed the letter before he became mayor of Boston, and several Republicans, including Reps. Paul Frost, Todd Smola, Donald Wong, Angelo D’Emilia, Shawn Dooley, Leonard Mirra, Steven Howitt, Peter Durant, Hannah Kane and Matthew Muratore.

Somerville Mayor Joseph Curtatone, Salem Mayor Kim Driscoll and Newton Mayor Setti Warren attended Monday’s State House meeting, a turnout that Garballey said was evidence that the project has attracted regional interest and support to help relieve congestion on the roads.

Eldridge said Warren spoke about how the North-South rail link would create workforce development opportunities for underserved communities. Attendees also received a presentation from Brad Bellows, the architect member of an advisory committee on the link appointed by Weld.

The project, should it ever come to fruition, is likely many years off as state officials are struggling to pay for a backlog of road, bridge and train infrastructure needs as well as other major projects like the Green Line Extension and commuter rail to the South Coast.

Garballey said he shares Baker’s concerns about costs, noting estimates over the years have ranged from $2 billion to $8 billion.

“We need to get a firm grasp on how much this is doing to cost and I think by doing the analysis we can have those questions answered,” he said.

Noting that Baker’s chief of staff Steve Kadish used to work for both him and Weld, Dukakis said persistence will be the key to getting Baker on their side.

“I’m convinced that we can persuade him that this makes a lot of sense,” he said.