LANSING – Michigan Attorney General Dana Nessel filed a lawsuit in Ingham County Circuit Court on Thursday that seeks to decommission the Enbridge Line 5 oil pipeline across the environmentally sensitive Straits of Mackinac.

If successful, the suit would block plans by Enbridge Inc. to dig a tunnel beneath the straits, which the company says would safely encase a replacement pipeline and remove the possibility of a catastrophic oil spill.

The new lawsuit — combined with one filed against the state earlier this month by Enbridge — also raises the possibility that the future of Line 5 could be tied up in years of litigation.

Nessel’s lawsuit alleges continued operation of the 66-year-old straits pipeline under the easement granted by the state in 1953 violates the public trust, is a public nuisance, and violates the Michigan Environmental Protection Act because it is likely to cause pollution impairment and destruction of water and other natural resources, her office said in a news release.

The suit seeks a shutdown of the line, which runs from Superior, Wisconsin, to Sarnia, Ontario, "after a reasonable notice period to allow orderly adjustments by affected parties."

Enbridge Line 5 is a 30-inch crude oil and gas liquids pipeline that splits into two 20-inch lines as it crosses the Straits of Mackinac.

“I have consistently stated that Enbridge’s pipelines in the Straits need to be shut down as soon as possible because they present an unacceptable risk to the Great Lakes,” Nessel said in the news release.

“Governor (Gretchen) Whitmer tried her best to reach an agreement that would remove the pipelines from the Straits on an expedited basis, but Enbridge walked away from negotiations and instead filed a lawsuit against the state. Once that occurred, there was no need for further delay.”

Enbridge spokesman Ryan Duffy said the company will need time to evaluate the court filing.

"We are disappointed the state chose not to accept our offer to advance talks on the straits tunnel, a project that would make a safe pipeline even safer," Duffy said in an email.

"The state also ignored our offer to suspend litigation and jointly appoint an independent, Michigan-based moderator to help facilitate the discussions. We also committed to making additional safety enhancements to the current line."

Duffy wouldn't say whether Enbridge would continue preparatory work for the tunnel while the lawsuit is pending.

Environmental groups and Democratic lawmakers applauded the move, while Senate Republican leaders called it "ill-advised and dangerous."

And Michigan Chamber of Commerce President and CEO Rich Studley blasted Nessel, saying she acted irresponsibly to fulfill a "reckless campaign promise," and that by saying in late May that she would move to decommission Line 5 in 30 days, she sought to interfere with negotiations that were then underway between Whitmer and Enbridge.

"We can only have one governor," Studley said. "It would appear that someone else thinks there are two, or that the governor works for the attorney general."

He added that Nessel has said she would not enforce laws she did not agree with. Michigan is "at risk of becoming a banana republic, where one person feels they are the executive, judicial and legislative branch, all rolled into one," he said.

In April, Nessel said she would never enforce a Michigan law, still on the books, banning abortion, if the U.S. Supreme Court overturns Roe v. Wade, the landmark case making the Michigan law inoperative.

In response to Studley, Nessel spokeswoman Kelly Rossman-McKinney said: "Corporate profit stopped trumping the public’s interest on Jan. 1," when Nessel took office.

She said it is not surprising Studley would "race to defend Enbridge," since the company donated more than $126,000 to a Michigan Chamber political action committee in the last election cycle.

"If Rich Studley wanted a voice in how the Department of Attorney General operates, he should have run for this office," Rossman-McKinney said.

In Whitmer's office, "we're not going to dignify that kind of nonsense with a response," said spokeswoman Tiffany Brown.

Sean McBrearty, coordinator of the Oil and Water Don't Mix steering committee, called Nessel's move "a dramatic turning point in the fight to save the Great Lakes from an Enbridge pipeline rupture."

Enbridge CEO Al Monaco has predicted gasoline, propane and jet fuel shortages in Michigan if Line 5 is shut down, along with price spikes that he would not quantify. He said experts agree pipelines are the safest way to transport hazardous liquids and a shutdown would result in more trucks and rail cars carrying oil.

About 22,000 Upper Peninsula households use propane as their primary source of heating. Those users, who get their propane from gas liquids extracted from Line 5 before it crosses the straits, are particularly concerned about potential spikes in the price of propane, which often hovers at around $2 per gallon.

Whitmer appointed a task force earlier this year to examine alternative propane sources for U.P. residents.

More:Shut down Enbridge Line 5? Here are 6 things to know

More:Enbridge seeks court ruling on Great Lakes oil pipeline deal

Enbridge filed a lawsuit in the Michigan Court of Claims on June 6, asking the court to rule on the legality of an agreement it reached with former Gov. Rick Snyder to build a utility tunnel beneath the channel linking lakes Huron and Michigan.

Nessel filed a response Thursday to that lawsuit, which names the governor as a defendant, asking for it to be dismissed.

Nessel has said a law enacted in December to implement the tunnel agreement violates the state constitution. The Canadian company wants the Court of Claims to rule on that issue, as well.

Whitmer had been negotiating with Enbridge officials on an accelerated timetable for digging the tunnel, which Enbridge would pay for at an estimated cost of $350 million to $500 million.

Whitmer said through a spokeswoman Thursday she has instructed the Department of Natural Resources to undertake a comprehensive review of Enbridge's compliance with the 1953 easement.

“Although the governor remains willing to talk with Enbridge, her commitment to stopping the flow of oil through the Great Lakes as soon as possible — and Enbridge’s decision to sue the governor rather than negotiate — will at some point require her to take legal action, as well," said Brown, Whitmer's press secretary.

Nessel and Whitmer are Democrats who both promised to shut down Line 5 during their 2018 campaigns.

After talks on the tunnel broke off, Monaco said the timetable Whitmer sought was not realistic. The governor wants the pipeline out of the water by 2021, but that's "not humanly possible," Monaco said.

Brown said Whitmer wanted an unspecified fixed date to get the pipeline out of the water, but Enbridge wanted to be "allowed to run oil through the Great Lakes indefinitely." Whitmer "has never viewed litigation as the best solution to this problem," she said.

Monaco said the tunnel, which would separate a replacement stretch of Line 5 from Great Lakes waters, will take four to five years to complete, at an accelerated schedule, and "we can't shut down that line before the tunnel is complete."

Snyder, a Republican who left office Jan. 1 because of term limits, struck a deal for Enbridge to build the tunnel shortly before he left office and pushed enabling legislation through the GOP-controlled Legislature.

Nessel issued an opinion in March that said the tunnel legislation violates the Michigan constitution because it did not have a single purpose described in its title, as the constitution requires.

In its lawsuit, Enbridge said "the state now refuses to honor the promises made in the December 2018 agreements."

Liz Kirkwood, executive director of the environmental group For Love of Water (FLOW), said Nessel is "upholding her public trust duty to defend the interests of the Great Lakes."

While Enbridge is touting the economic importance of Line 5, Michigan residents and leaders need to be mindful of the hundreds of thousands of jobs tied to the Great Lakes and how tourism and other industries would be devastated by a spill, Kirkwood said.

She said Enbridge's safety record is not a good one, recalling the 2010 incident when a break in another Enbridge line spilled more than a million gallons of crude oil into the Kalamazoo River in western Michigan. The cleanup cost Enbridge well over $1 billion.

Sen. Jeff Irwin, D-Ann Arbor, acknowledged that the court fight could be "long and arduous," but said that only demonstrates "we should have started this process long ago."

Senate Majority Leader Mike Shirkey, R-Clarklake, issued a joint statement with Sen. Wayne Schmidt, R-Traverse City and Sen. Ed McBroom, R-Vulcan. They said Nessel "is slowing progress on a tunnel — not a penny of which will be paid for by taxpayers — that will ensure the continued safety of a vital energy conduit."

If the line is shut down, "Michiganders will pay significantly more for gas at the pump and for propane to heat their homes," the senators said. "She will put at risk thousands of jobs that are directly related to the line, and thousands more that would be created by the tunnel project."

Duffy, the Enbridge spokesman, said the company remains open to talks with Whitmer and believes the tunnel "is the best way to protect the community and the Great Lakes while safely meeting Michigan’s energy needs."

He said the pipeline is "critical infrastructure that Michigan residents depend on every day, and it would be irresponsible to shut it down." The pipeline is "safe and well maintained, and we intend to continue to operate it for decades to come," he said.

Contact Paul Egan: 517-372-8660 or pegan@freepress.com. Follow him on Twitter @paulegan4. Read more on Michigan politics and sign up for our elections newsletter.