It seemed simple enough: Ahead of a pivotal vote on a controversial project, the League of Women Voters Newton invited opposing sides to make their cases to residents at a public forum on Thursday.

But two days before the event, a local group opposing Northland Investment Corp.’s development in Newton Upper Falls declined to appear.

The result leaves the forum — intended for voters to hear two perspectives on the project — decidedly one-sided.

“After serious consideration, we regret that we will not be participating in the League’s event,’’ said a statement released Tuesday by the Committee for Responsible Development, a ballot question committee working against the project. “The League’s format cannot be regarded as a fair and open forum for debate.’’

Despite the absence, the forum will move forward, the League said in a statement.

The League regrets the group has decided to withdraw from the event, but “will proceed as planned in fairness to the other participants and those planning to attend,’’ according to the statement.

Northland’s mixed-use project, approved by the Newton City Council in December, would erect 14 buildings on about 22 acres at Needham and Oak streets, including 800 apartments, 180,000 square feet of office space, and 115,000 square feet for retail and community space.

Opponents of the project, who say it is too large and will create too much traffic, launched a successful effort to put the issue before voters in a special election timed with the presidential primary on March 3. (Technically, Newton voters will decide whether to rescind zoning changes needed for the project to move forward.)

Northland’s proponents say the project will bring more affordable housing and economic benefits to the city; the developer has also pledged to include money for a school project, a shuttle service, and a traffic management plan for the site.

The Northland project is backed by the city’s mayor; the city’s Democratic Party; and local housing, business, and environmental advocates.

The League endorsed the Northland proposal before its approval by city councilors, and scheduled Thursday’s forum ahead of the March 3 referendum vote. Along with the Committee for Responsible Development, which opposes the project, the League invited another local ballot committee, Yes for Newton’s Future, which supports the development.

The League of Women Voters Newton, a nonpartisan grass-roots political organization, routinely organizes candidate debates and forums on public issues. It scheduled the forum for Thursday, Feb. 13, at the Durant-Kenrick House, 286 Waverley Ave., starting at 7 p.m.

The League invited representatives from both sides of the Northland issue “consistent with its long-standing practice," according to its statement. The goal of the forum is to “provide information to residents in order to help them to make an informed decision,’’ it said.

The Committee for Responsible Development, which opposes the project, said the League was “not a neutral, objective body’’ regarding the Northland development. But it still looked forward to participating in “a fair and open forum.’’

Ultimately, however, the group objected to the format, the choice of moderator, and how information about the project would be presented at the event.

“I’m feeling that our arguments wouldn’t be fairly heard, and that is very important to us,’’ said Martina Jackson, a cochairwoman of the ballot committee, in an interview. “It would be a forum for them, and we would be there as sort of a foil."

Jackson said the group objected when the League sought a project supporter to serve as moderator and to pick questions from audience members.

Allison Sharma, chairwoman of Yes for Newton’s Future, said her group was disappointed by the news, because “we believe the more Newton voters learn about the Northland project, the more likely they are to support this tremendously thoughtful, well-balanced project.’’

Both sides on the issue have launched get-out-the-vote efforts, including sending volunteers door to door to hopefully sway voters — who may otherwise be focused on presidential politics — before Super Tuesday. The forum is an other chance to reach voters.

“We were looking forward to this opportunity to share the many benefits of the project, including affordable housing, open space, and environmental sustainability,’’ Sharma said.

The Northland project has also touched off a debate that frames the outcome of the March 3 vote as an indicator of what kind of development the city will encourage in the future.

This isn’t just about Northland or Upper Falls, said Jackson from the opposition group.

“This is precedent,’’ Jackson said. "If this passes, it will give developers and the City Council a green light to go and develop anywhere, and disregard the community.’’

John Hilliard can be reached at john.hilliard@globe.com.