PORTLAND

Environmental groups are digging in for a long fight against a federal lawsuit that seeks to overturn South Portland’s ban on loading crude oil onto tankers at the city’s waterfront.

The Portland Pipe Line Corp. sued the city in U.S. District Court in Portland last week over the ban adopted last year. The city ordinance was designed to prevent the South Portland-to-Montreal oil pipeline from being reversed, which would allow Canadian oil to come into Maine.

Residents and environmentalists have expressed concern about the potential importation of tar sands oil, which they say is dangerous to ship and difficult to clean if spilled. They suspect Portland Pipe Line wants to bring the oil from Canada then load it onto tankers at South Portland to ship to buyers elsewhere.

Portland Pipe Line officials said last year the company doesn’t have plans to reverse the pipeline, but they haven’t said why the company wants to put crude on tankers at South Portland. The lawsuit says only that the city’s ordinance “contravenes fundamental principles upon which our Republic was founded.”

Members of environmental groups said Thursday they want to help the city defend the ban. They said they are planning to file court papers to join the lawsuit once the city responds to it.

Taryn Hallweaver, director of Environment Maine, said the pipeline’s lawsuit seeks to “reverse the will of the people,” and her group plans to assist South Portland in fighting it. Mary Jane Ferrier, a spokeswoman for Protect South Portland, agreed, and said she suspects the city and its allies are in for a drawnout battle.

Other environmental groups that have said they would sign on to fight the pipeline lawsuit are the Conservation Law Foundation and Natural Resources Council of Maine.

South Portland city officials did not return phone calls seeking comment. Jim Merrill, an attorney for the pipeline company, declined to comment Thursday.

The pipeline pumps crude from South Portland to Montreal, crossing New Hampshire, Vermont and Quebec. Opponents of the city’s say the environmental concerns are overstated and the prohibition on loading tankers will end up costing jobs.

Patrick Woodcock, director of the governor’s energy office, said last year he hopes the state can craft a way to manage the flow of petroleum products instead of having it decided on the local level. He did not immediately return a phone call seeking comment Thursday.