LANSING – The fight between Enbridge and the State of Michigan over the future of the Line 5 oil pipeline appears headed for the courts, but it was unclear Monday whether the Canadian energy giant or Gov. Gretchen Whitmer would make the next move

Whitmer, who promised during her 2018 campaign to shut down the Enbridge Line 5 pipeline through the Straits of Mackinac, is "reviewing all ... options" about what to do next, a spokeswoman said Monday.

Tiffany Brown made her comments after Whitmer on Thursday ordered state agencies to halt work on a proposed tunnel to encase a replacement for the 66-year-old stretch of pipeline that carries crude oil and natural gas liquids through the environmentally sensitive straits.

Whitmer's directive came soon after the release of an opinion she requested from Attorney General Dana Nessel, a fellow Democrat who also campaigned on shutting down Line 5. Nessel said legislation to allow for construction of the $350 million to $500 million tunnel, approved during the lame-duck session during the last days of the administration of Republican Gov. Rick Snyder, violates the state constitution.

Proponents see the tunnel, to be dug into bedrock beneath the straits, as the best way to keep oil flowing and protect against a spill caused by an anchor strike or a crack in the pipe. Critics say the line mostly moves Canadian oil to Canadian markets, there are other ways to deliver propane to Michigan's Upper Peninsula, and the Great Lakes would continue to face unacceptable risk of a catastrophic spill during the roughly 10 years it's expected to take to construct the tunnel.

What happens next?

Observers are split over whether Enbridge or Whitmer and Nessel are likely to make the next move.

Enbridge could go to court — likely the Michigan Court of Claims — to seek a ruling that Public Act 359 of 2018 and Enbridge's tunnel agreements with the state and the authority are lawful and valid, contrary to Nessel's ruling.

Whitmer could first move to shut down the pipeline.

Most of the key parties were evaluating their options Monday and not saying much.

The Mackinac Straits Corridor Authority — the body created by the 2018 legislation to oversee tunnel construction, whose existence was declared null and void by Nessel's opinion — appears to be in no position to act, said Chairman Mike Nystrom.

Why does Nessel say the legislation is unconstitutional?

Nessel's concern relates to compliance with the following requirement in the Michigan constitution:

"No law shall embrace more than one object, which shall be expressed in its title. No bill shall be altered or amended on its passage through either house so as to change its original purpose as determined by its total content and not alone by its title."

Republican lawmakers were rushing to pass the tunnel legislation during the lame-duck session and their work was complicated by another constitutional requirement that no bill can pass until it has been in each chamber for at least five days.

To get around that requirement and save time, lawmakers used a "vehicle bill," introduced earlier in the Senate and related to the Mackinac Bridge Authority, to create the new Mackinac Straits Corridor Authority. That maneuver, and the resulting complications with the title and purpose of the bill, are central to Nessel's constitutional objections.

Could the courts overturn Nessel's ruling?

That remains to be seen.

A principle of Michigan law, referenced in Nessel's opinion, is that courts have a duty to construe a law as constitutional "unless its unconstitutionality is clearly apparent."

More:Attorney General Dana Nessel: Legislation to allow Line 5 tunnel is unconstitutional

More:Snyder signs Line 5 tunnel bill, makes appointments, with record speed

At the same time, Michigan courts have previously overturned Michigan statutes for violating the same section of the constitution cited by Nessel. Among the concerns is that lawmakers know by the title what they are voting on.

In this case, "the people in the chamber knew what they were voting on," House Speaker Lee Chatfield, R-Levering, said Monday on the Frank Beckmann Show on WJR-AM.

In an earlier case that placed great emphasis on the separation of powers between Michigan's legislative, judicial and executive branches, Michigan courts gave deference to actions by the Legislature that did not appear to comply with a separate requirement of the Michigan constitution.

The constitution requires a two-thirds vote in each chamber for a bill to be given immediate effect, but the state House routinely gavels through approval of immediate effect by a voice vote, over the objections of minority members, even when the bill was approved by a narrow vote.

In 2012, House Democrats went to court, asking how Republicans who control the chamber could be complying with the two-thirds requirement if they don't actually count the votes.

"How can this court or any court interfere with the inner workings of a legislative body?" asked Michigan Court of Appeals Judge Michael Riordan, before the three-judge panel ruled to uphold the practice.

The Michigan Supreme Court declined to hear the case.

What could Whitmer and Nessel do?

The governor and attorney general could move to revoke the 1953 easement, granted by the state, that Enbridge relies on to legally cross the Straits of Mackinac with its pipeline.

Among other terms, the state requires Enbridge to "exercise the due care of a reasonably prudent person for the safety and welfare of all persons and of all public and private property."

The state can give Enbridge written notice that it is in violation of one of the terms of the agreement, giving the company 90 days to get to work on rectifying the problem or face revocation of the agreement.

Enbridge was earlier notified of violations related to the amount of spacing between pipeline supports, in both 2014 and 2016.

Liz Kirkwood, an attorney and executive director of For Love of Water in Traverse City, said she has no knowledge of what Whitmer or Nessel are considering, but she believes they could move to revoke the easement without calling for remediation, based on those past notices and other areas of noncompliance.

Environmentalists such as Kirkwood say the need to do so is highlighted by a federal court ruling Friday that identified significant weaknesses in Enbridge's spill response plans and the way those plans were approved by a federal agency.

"Although one has never occurred, an oil spill in the Straits of Mackinac poses a significant threat to Lake MIchigan and Lake Huron," said U.S. District Judge Mark Goldsmith, in sending the response plan back for more work.

The Enbridge easement does not set out a specific notice requirement for revocation, but Whitmer would likely want to make sure she has alternative plans to supply affected customers, including residential propane users in the Upper Peninsula and the Marathon oil refinery in Detroit.

What would happen if Whitmer revoked the easement?

Enbridge would almost certainly challenge the revocation in court.

The situation is complicated by agreements Snyder negotiated with Enbridge in 2018 that say Enbridge is in compliance with the easement.

Kirkwood said Whitmer and Nessel could argue those agreements are invalid because they don't comply with the Great Lakes Submerged Lands Act. Among other shortcomings, the state didn't properly evaluate risks and alternatives or hold public hearings before signing the agreements, she said.

Is a compromise possible?

Among the concerns when the Legislature passed the tunnel legislation last year was the length of time the construction was expected to take and what many lawmakers saw as inadequate insurance coverage or bonding by Enbridge in the event of a spill.

It's not clear whether Enbridge could accelerate the time line, increase bonding, or make other changes that would lessen opposition.

The situation is complicated for Whitmer and Nessel because some labor groups — a key Democratic constituency — support the tunnel project as an economic development engine.

"When she looks at the whole picture, I think she'll make the right decision, which is allowing the tunnel to go forward," said Pat Devlin, secretary-treasurer of the Michigan Building and Construction Trades Council.

But David Holtz, conservation chairman at the Michigan chapter of the Sierra Club, said the pipeline's benefits to Michigan are too sparse and the consequences of a spill would be too devastating to justify anything short of a pipeline shutdown.

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.