Mackinac Island — Attorney General Dana Nessel said Wednesday she will act by the end of June to shut Enbridge's Line 5 oil pipeline beneath the Straits of Mackinac if Gov. Gretchen Whitmer doesn't reach an agreement on a tunnel to house the pipelines.

"If they cannot come to an agreement that allows for a quick process of the decommissioning of Line 5 then I will act," Nessel said in an interview on the sidelines of the Mackinac Policy Conference. "I'm not prepared to wait much longer on that."

Nessel said she and the governor "share a deep concern" for the Great Lakes and she'd like to give Whitmer the opportunity to craft an agreement that accelerates the decommissioning of Line 5. But the Plymouth Democrat said she has a duty to defend the state Constitution, which provides for the protection of natural resources.

"Every day that Line 5 continues to operate is a day that our state is in great peril and that we could possibly see what could be the largest oil spill in American history," Nessel said.

There are several legal mechanisms Nessel's office could use to close the pipeline, she said.

Nessel declined to say which route her office would choose. But on the campaign trail last year, she said she could bring a case in the state Court of Claims to allege the condition of the pipelines violated Enbridge's easement in the Straits.

Line 5 transports about 23 million gallons of oil and natural gas a day through the Upper Peninsula, including a four-mile stretch through the Straits of Mackinac.

Whitmer continues to discuss the path forward with Enbridge and stakeholders, the governor's spokeswoman Tiffany Brown said Wednesday.

"As you know, the governor is committed to a solution that protects the Great Lakes, removes the pipelines from the Straits as soon as possible and provides for the Upper Peninsula’s energy needs," Brown said.

The Canadian pipeline company remains in talks with the Whitmer administration, Enbridge spokesman Ryan Duffy said Wednesday.

"Enbridge is committed to continuing to safely and reliably deliver the energy Michigan needs," Duffy said. "That is why we are investing $500 million to construct a tunnel to house a new Line 5 at the Straits of Mackinac; an option supported by more than half of all Michiganders, according to a recent survey commissioned by the Michigan Chamber of Commerce."

In late March, Nessel in a formal legal opinion declared unconstitutional the law passed by the Republican-controlled Legislature in December creating an authority to oversee construction of a tunnel to house Line 5 and other utilities. She argued the law was invalid “because its provisions go beyond the scope of what was disclosed in its title.”

Whitmer quickly ordered state agencies to halt their work on the project based on Nessel's opinion.

Senate Majority Leader Mike Shirkey, R-Clarklake, has said Nessel's opinion is "wrong" and the attorney general "doesn’t have the power of scrutiny over constitutional issues. That’s why it’s called an opinion."

House Speaker Lee Chatfield, R-Levering, has called Nessel’s opinion “laughable,” contending that lawmakers knew what they were voting on even if the title object clause in the legislation was mishandled.

Enbridge, Line 5's owner, is a sponsor here at the Detroit Regional Chamber's conference, where lawmakers and businesses have gathered. As attendees loaded onto the ferry Tuesday to traverse the Straits, oil pipeline opponents protested outside the Shepler's ferry dock and are expected to protest Wednesday in the island's downtown.

Environmental activists have long called for Line 5 to be shut down, warning an accident like the 2010 oil spill in the Kalamazoo River could devastate the Great Lakes.

Last year, an apparent anchor strike released roughly 600 gallons of dielectric fluid, a type of mineral oil, from utility lines owned by Pewaukee, Wisconsin-based American Transmission Co.

The incident lent urgency to discussions around the pipeline and at the end of his term last year, former Gov. Rick Snyder finalized an agreement with Enbridge to construct a shared utility corridor beneath the straits to house the pipeline.

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