With half their case already dismissed, a group of plaintiffs vying to stop the City of Boston’s public-private plan to rehab White Stadium for a pro soccer team made a last-minute motion near the end of trial to try to enhance their legal claims.

The late motion aims to provide a layer of insurance to the plaintiffs’ remaining major claim, by way of a legal theory purportedly backed by state law that gives 10 taxpayers the ability, through the court, to block the type of project the city and Boston Unity Soccer Partners are pursuing with their plan to rebuild the stadium on public parkland.

While 20 neighbors of Franklin Park’s White Stadium joined the Emerald Necklace Conservancy in filing last year’s lawsuit, not all of them were homeowners who pay property taxes. The motion adds to their legal challenge, that the proposed for-profit stadium would illegally privatize protected public land, by ensuring that the plaintiffs include 10 “taxable inhabitants of the City of Boston.”

“Under this statute, this court ‘shall have jurisdiction in equity, upon petition of not less than ten taxable inhabitants of the city or town in which such common or park is located, to restrain the erection of a building on a common or park in violation of this section,’” the motion filed Wednesday by Attorneys Alan Lipkind and Nicholas Allen states.

The motion asserts that the city is violating state statute by erecting a more than 600,000 square-foot building on designated public parkland without legislative approval, and by raising funds for the roughly $200 million project.

That legal argument forms the basis of the plaintiffs’ remaining case, which is that the public-private plan violates Article 97 of the state constitution, which voters approved in 1972 and requires two-thirds approval from the state Legislature for other uses for land and easements taken or acquired for conservation purposes. The city and BUSP deny the privatization claim.

The plaintiffs’ motion “to conform their pleadings to the evidence presented at trial,” drew backlash from attorneys for the City of Boston, who argued that the last-minute legal maneuver should not be allowed by Suffolk Superior Court Judge Matthew Nestor.

An attorney for the city, when the motion was introduced at the trial Wednesday, described it as “extremely prejudicial to us,” given that the plaintiffs were seeking to “add claims in the middle of a trial” that the city’s legal team had not had a chance to review nor prepare evidence for.

Lipkind responded by saying there “no new claims” introduced by the motion.

“It’s just another tool to give the court a legal theory to rely on,” Lipkind said.

A city attorney also sought to convince Nestor to disallow the motion by arguing that it was “futile,” given that the city is only paying for its half of the project, there’s “nothing illegal” about the city spending city funds to build a school building and sports stadium, and Boston Public Schools will retain ownership of White Stadium after it’s rebuilt.

The city’s legal team also argued that the proposed use fits under the “works of beauty and public utility” for Boston residents category that the municipality was authorized to use Franklin Park for, when it was purchased by a public charitable trust in 1947 for the purpose of establishing a stadium there.

The plaintiffs’ half of the case arguing that the trust, the George Robert White Fund, does not allow for “joint undertakings” such as what the city and Boston Unity is proposing, was thrown out by Nestor on the eve of trial Monday.

In this instance, Nestor, who ruled in favor of the city on all pre-trial motions, opted to allow the plaintiffs’ last-minute motion on Wednesday. He had also rejected the city and BUSP’s motion to dismiss the plaintiffs’ claims a day earlier, allowing the trial to continue.

Nestor said that while it’s “always good to bring claims before” a trial, the law is clear that there’s nothing barring a new claim in a civil case … “even though it’s late, really late.”

“But it’s not too uncommon,” Nestor said. “I will allow the motion.”

The matter came up again at the end of the day’s trial session, when Gary Ronan, an attorney for the city, told Nestor that the city’s legal team needed more time “to address the amended complaint.”

Nestor, who had said earlier that he would allow the defendants the ability to address the amended claim by introducing new evidence or a new witness, said that he wouldn’t allow much more time, given that he was looking to wrap up the trial with closing statements on Thursday.

“You know what the theory is so nothing from here on out should be a surprise,” Nestor said. “If there’s anything in the written amended complaint that creates something different, I’ll certainly reconsider it.”

The second day of trial revolved around the city’s witnesses, two city officials who were involved in what they described as an extensive review and design process for the White Stadium rehab and a BPS athletics official who spoke of how he felt the plan would bring much-needed improvements to the run-down 76-year-old facility.

While Nestor ruled against allowing the plaintiffs’ pre-trial motion to bring forward public drinking concerns with the plan, he opted on Wednesday to allow their attorneys to raise the issue of state statute not allowing alcohol on school property.

Nestor, after a city attorney objected, said the city’s legal team opened that door when questioning a witness about permitted events at the stadium, which BPS owns and would share use of with the  National Women’s Soccer League expansion team.

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The city’s contention that its prior classification of Franklin Park as being protected by Article 97 — a key point of contention in the plaintiffs’ case — was due to a mapping mistake by a retired Parks Department employee was raised again during testimony by Interim Parks Commissioner Liza Meyer.

The issue, regarding classification in city open space plans that go back “decades,” was first raised Tuesday in opening statements by the plaintiffs’ and city’s attorneys.

Day 3 of the high-stakes trial, which will determine the fate of the controversial plan championed by Mayor Michelle Wu, will convene at 9 a.m. Thursday. Wu’s opponent in the mayoral race, Josh Kraft, has called for a pause on the project until the litigation is resolved.