Calgary-based Enbridge Inc.’s Line 5, a 540,000 b/d oil pipeline, Thursday has temporarily survived the opening legal shots in a revived Michigan campaign to shut it down by demolishing its four-mile underwater leg across the Straits of Mackinac.

“Line 5 will continue to safely operate as it has for more than 60 years,” said Enbridge management in response to what Republican opponents called legal hair-splitting by Gov. Gretchen Whitmer and Attorney General Dana Nessel, both Democrats, who took office in January.

Whitmer, acting on a memorandum by Nessel, ordered Michigan agencies on Thursday to stop all actions regarding the tunnel. Last fall Enbridge and the state’s former Republican executive branch, led by Gov. Rick Snyder, had agreed that Enbridge would cover the costs of an underwater tunnel to accommodate a replacement for its 65-year-old liquids system.

The contested agreement was a compromise solution to resolve disputes over environmental hazards posed to the Great Lakes region by the old pipeline. Enbridge committed to pay an estimated $500 million for the tunnel and to issue a $1.87 billion bond to cover damages if the old water crossing leaks before the new one was completed.

However, Whitmer used a legal opinion issued by Nessel to halt work on the tunnel replacement. The decision is expected to pit the administration against a Republican-control legislature.

Nessel said in the opinion that the law creating the authority to approve the agreement with Snyder was unconstitutional “because its provisions go beyond the scope of what was disclosed in its title.”

Whitmer’s executive order directed all state departments to stop any activity related to the tunnel project and "report to the governor's legal counsel regarding actions taken since the bill was passed." If a court ruling were to find the law unconstitutional, it would be applied retroactively, voiding prior actions, including those by the Snyder administration, Nessel said.

Whitmer and Nessel each had campaigned for office last fall on promises to shut down the Straits of Mackinac pipeline crossing.

Republican leaders in the Michigan legislature called the strategy legal hair-splitting that would be challenged in court.

“We intend to seek clarification from the administration on a path forward,” Enbridge said of Whitmer plans. “Enbridge worked in good faith with the Michigan government on the tunnel project.”