SOUTH PORTLAND — A citizen group opposed to the prospect of Canadian tar sands oil being pumped to South Portland for shipment overseas said Thursday it will try to change a city zoning law to prevent any such project from coming to fruition.

At a rally of a couple dozen supporters outside South Portland City Hall, leaders of the Concerned Citizens of South Portland unveiled the revised zoning ordinance and began gathering signatures to place a question for its approval by voters on the November ballot.

While there is no current plan to reverse the flow along 236 miles of the buried pipeline and use it for oil exports, critics say the company is positioning itself to do so in the future, making the ballot initiative the best hope to preclude such a plan before work ever would get underway.

“South Portland is the only city or town where we have any leverage,” said Roberta Zuckerman, a member of the citizen group. “There is a lot resting on what South Portland does at this time.” In a statement, Portland Pipe Line Corp. spokesman Ted O’Meara said the company will in the near future respond to the “misinformation, exaggerations and mistruths (sic)” propagated in the ballot petition.

O’Meara said the company has been a consistent provider of local jobs and a contributor of about $25 million in property taxes over the last three decades. It also has granted the public access to 5 acres of what is now Bug Light Park and the green-belt walkway, evidence of its community involvement.

O’Meara said that if the company decides later to pursue expansion or development of its waterfront property, it would engage with authorities and comply with regulations.

“Should we decide to develop our facilities further we have every confidence that the City of South Portland and other agencies would treat us fairly and consistently as they have in the past,” O’Meara wrote. “In the meantime, we ask that the community not discriminate against one of the top taxpayers and job creators in the city.”

If passed by voters, the zoning provision could spark a legal challenge, placing South Portland at the center of an international struggle about Canada’s ability to export and capitalize on the valuable tar sands resources buried under Alberta. To get the issue on the ballot, volunteers said, they must collect 950 validated signatures of registered South Portland voters. The citizen group hopes to turn them in to the city clerk by June 17. The deadline is not strict, but would leave the City Council and the Planning Board ample time to evaluate the proposal, said Emily Figdor, director of Environment Maine, which collaborated with the citizens’ group on the ballot initiative.

“We worked really hard (so) that all of our ducks were in a row, and something like that takes time,” Figdor said. “We wanted to do it right.”

Because the matter is framed as a zoning issue, Concerned Citizens co-chairman Robert Sellin said, the city would be on firm legal footing if petroleum or pipeline companies sue the city.

He said the group’s proposal aligns with a comprehensive plan passed in 2012 that calls for the city to phase out heavy industrial uses in the waterfront area.

The zoning provision, or “Waterfront Protection Ordinance,” as dubbed by Sellin’s group, would define and specifically allow the pipeline’s current use (moving imported oil from a shipping terminal in South Portland 236 miles north to refineries in Montreal). A separate section would bar the expansion, construction or installation of new equipment that would be used for the loading of oil onto tankers for export from South Portland to overseas refineries.

Jeff Thaler, visiting professor of environmental law at the Maine Law School and the University of Maine, said there is a body of case law in which the state’s Law Court has upheld cases of ballot initiatives used to target specific developments. Thaler, who is not connected to the project and spoke without specific knowledge of the case, predicted that the Portland Pipe Line Corp. soon will begin mounting a public information campaign to sway opinion. At least one resident has said that an unidentified group already has begun polling on the issue, the Portland Press Herald reported in April.

“The developer is going to have to defeat the initiative, or else they’re going to have to figure out a way around what the initiative is doing, or figure out a way not to go through South Portland,” he said.

“It certainly has some precedent here in Maine and can be very frustrating for a developer who spends a lot of time and money only to have the rules change before the end of the game.”

At the rally, air pollution and the risk of a spill of the thick tar sands crude were among the loudest concerns.

When the Portland Pipe Line Corp. proposed reversing the flow in 2008, the project included installing two 70-foot smoke stacks. While the project was taken off the table in 2010, opponents on Thursday still pointed to the stacks as potential sources of greenhouse gases and other pollutants, not to mention their presence as unsightly additions to the view from nearby Bug Light Park.

The stacks would be needed to burn off chemical gases used to prevent explosions or fire in the loading of oil products into tanks.

Three similar stacks now stand in South Portland to accommodate the loading of gasoline, heating oil and jet fuel. Also, the relative amount of new emissions from the two towering structures would have been low compared to other pollutant sources in the area.

The Portland Pipe Line Corp. already is permitted to expel 220 tons of volatile organic compounds, which are believed to cause eye, nose, and throat irritations and are suspected of causing cancer in some people. Their 2008 permit for the project that was never completed, meanwhile, allowed them to add 39 more tons, less than 20 percent of the overall permitted amount. By comparison, the Sappi paper mill in Westbrook may legally emit 321 tons of volatile organic compounds, while Bath Iron Works is allowed to expel 102 tons of the substances.