People walk into the Portland Museum of Art in Portland. At left is the former children’s museum at 142 Free St., which is at the center of a public debate that the City Council will weigh in on Monday. Gregory Rec/Staff Photographer

Opponents of the Portland Museum of Art’s proposed demolition of the former children’s museum said Thursday they are prepared to continue fighting if Monday’s City Council vote doesn’t go their way.

Speakers at a news conference organized by Greater Portland Landmarks stopped short of saying they might sue the city if the demolition is approved. But a lawyer for the nonprofit preservation group said it would be a “legal error” for the council to allow the demolition for reasons that aren’t outlined in the city’s historic preservation ordinance.

“We need to see exactly how they rule before we really know what we might do next,” said Carol De Tine, board vice president of Greater Portland Landmarks, the preservation group that has been leading opposition to the demolition. “I don’t think we’ll just lick our wounds.”

The museum is asking the city to remove the historic designation that protects the building at 142 Free St., which the museum owns and is located next door. Tearing down the brick building is part of the PMA’s planned expansion to more than double the museum’s size. Built in 1830 and remodeled by noted architect John Calvin Stevens in 1926, the building has been home to the local Chamber of Commerce and the Children’s Museum and Theatre of Maine.

The council first took up the matter at its May 6 meeting, when a majority of members seemed to support removing the designation, but it postponed the decision until this Monday’s council meeting. Both the city’s Historic Preservation Board and planning board have recommended against the designation change that would allow demolition.

Many of the points that demolition opponents brought up Thursday have been made previously during the public debate – that the building meets the requirements for protection under the city’s historic preservation ordinance, that rescinding it could affect the federal tax credits available to developers rehabbing properties within the Congress Street Historic District, and that the museum has enough property to expand without the tear-down.

A lawyer representing Greater Portland Landmarks added that the council cannot base its decision on whether the demolition is good or bad for the city and its future.

Sean Turley, of the law firm Murray Plumb & Murray, said the city’s historic preservation ordinance requires the council to weigh the recommendations of the Historic Preservation Board and planning board and decide the matter in a “quasi-judicial manner.”

“The ordinance expressly says what you can consider. You can’t consider the (city’s) comprehensive plan or any other policy,” Turley said. “Their only task really is to follow the ordinance. So if they deviate from that, that is obviously not following the law.”

Graeme Kennedy, the Portland Museum of Art’s creative director and director of public relations, would not say specifically what the museum might do next if the city ultimately blocks the demolition of 142 Free St. or how its expansion plan would be affected.

“We are looking forward to Monday and are taking this process one step at a time. We believe the city has the opportunity to grow through our inclusive Art for All mission and $100 million investment in downtown Portland,” Kennedy said in an email.

The debate over whether the museum should be allowed to tear down 142 Free St. has been heated and contentious, prompting many letters to the editor in the Press Herald from both supporters and opponents.

Kennedy tried to attend the news conference Thursday, at Greater Portland Landmarks’ office on Congress Street, to hear what opponents would be saying to the media. He said he had been made aware of the event just a few minutes earlier, by media members who called the museum seeking interviews.

But news conference organizers said the event was for media only and asked Kennedy to leave.

“I was only trying to stay informed so that the museum could respond appropriately after their media alert,” Kennedy said in his email.

The news conference Thursday lasted about an hour and featured three main speakers: De Tine; Sally Oldham, one of the group’s advisory trustees and a former vice president of the National Trust for Historic Preservation; and Scott Hanson of Hanson Historic Consulting in Topsham.

The Portland City Council is scheduled to meet Monday at 5 p.m. at City Hall.