SOUTH PORTLAND — The City Council voted Monday night to ask voters in November whether they want an ordinance to pre-emptively prevent so-called tar sands oil from flowing through the city.

“It’s now in the citizens’ hands,” said Mayor Tom Blake after the vote around 11 p.m.

The council could have approved the ordinance itself, but chose instead to put it on the Nov. 5 ballot.

The 5-1 vote, with one councilor absent, sets up a referendum campaign that will pit environmentalists against the multimillion-dollar oil and gas interests that have dominated South Portland’s waterfront for decades.

“There may be multiple lawsuits against the city of South Portland,” said Councilor Gerard Jalbert, the first to speak after nearly four hours of public comments.

Jalbert said he would vote for the ordinance, but ultimately he voted against its approval.

“Nov. 5 represents the first mile of a marathon,” Jalbert said.

The first to voice opposition to the ordinance was Councilor Linda Cohen, who reiterated the argument by marine businesses and oil and gas interests that the restrictions would have the unintended effect of shutting down companies that ship and handle a wide variety of oil and gas products that are bought and sold all over northern New England.

“This ordinance just goes way too far,” Cohen said.

Before the council’s brief discussion, hundreds of the ordinance’s supporters and representatives of the oil and gas industry packed the sweltering auditorium at Mahoney Middle School to argue their case. More than 60 spoke before the council.

South Portland is a vital port for shipments of petroleum bound for refineries in Montreal, and is considered an outlet for regional commerce, as one of the only ports with year-round deep-water access.

There is no current proposal to reverse the flow of the 236-mile pipeline operated by Portland Pipe Line Corp., which now sends crude oil from South Portland to Montreal.

But opponents of tar sands oil fear that plentiful reserves in western Canada will change that, with South Portland becoming a destination for the oil, which would be exported from the port.

The ordinance would restrict petroleum-related activity to the offloading of ships and would prohibit expansion of petroleum-related activity.

Tar sands refers to the raw, dense form of petroleum called bitumen that is mixed with sand, clay, and water in underground deposits. To process the substance into usable products, all of the non-petroleum elements must be removed after extraction.

The remaining bitumen at room temperature has the consistency of cold molasses, and must be heated or diluted with chemicals for it to flow through a pipeline. The bitumen must also undergo a preliminary refinement before it is considered to be a form of crude oil.

Proponents say the proposal to bar any potential project to reverse the pipeline’s flow to bring tar sands through the city is the only way to protect the city from the risk of any future spill.

In South Portland, six large oil terminals generate about $1.6 million in annual tax revenue for the city and represent about 2.7 percent of the city’s tax base.

Oil and gas interests say the ordinance would lead to the elimination of petroleum handling in the city. There are nearly 100 petroleum storage tanks, which dominate parts of the landscape near South Portland High School and along portions of the waterfront.

The council’s discussion followed two hearings by the Planning Board, which had to determine whether the ordinance is consistent with the city’s comprehensive plan. In a 4-2 vote, the board found that the ordinance does not meet with the city’s vision that parts of the working waterfront be maintained and improved.

South Portland could play a key role in an international struggle over Canada’s ability to export the substance, with the Portland Pipe Line viewed as one of the possible ways for Canada to export the oil, although the company that owns it has not made a proposal to do so.

Matt Byrne can be contacted at 791-6303 or at mbyrne@pressherald.com