CONCORD, N.H. – A top government lawyer ignored historical evidence and relied on “completely implausible” arguments in supporting Maine’s claim to the Portsmouth Naval Shipyard, New Hampshire’s attorney general said Thursday.

The shipyard, on an island in the river that separates the two states, is at the center of a decades-old border dispute that New Hampshire hopes the U.S. Supreme Court will settle once and for all. Maine, with the support of the U.S. Solicitor General, wants the case thrown out.

New Hampshire contends the boundary rests along the shoreline of Maine, while Maine says the interstate border has always been determined to be the middle of the river, putting the island squarely in Maine.

Last month, Solicitor General Seth Waxman, who represents the nation’s interests before the U.S. Supreme Court, sided with Maine and recommended dismissal of the case.

Waxman argued that a decree signed by King George II in 1740 clearly established the boundary as the middle of the river and that a 1976 U.S. Supreme Court ruling reinforced that line. Taking those two orders together make it unnecessary to look further, Waxman said.

But New Hampshire Attorney General Philip McLaughlin argues Waxman was too shortsighted.

In a brief filed with the court Wednesday, McLaughlin said the 1740 decree may be relevant, but only if considered along with historical evidence that New Hampshire operated, regulated and controlled the river.

McLaughlin said Waxman’s argument rested on interpreting the 1740 decree based on principles that weren’t used until decades later.

“The interpretation suggested by the United States is completely implausible,” McLaughlin said. “Whatever the 1740 decree may mean, it cannot possibly have been intended to designate a location whose significance depends entirely on navigational aids installed more than a century later.”

New Hampshire also argues that the 1976 order addressed a separate issue and did not preclude either New Hampshire or Maine from later disputing the river boundary, McLaughlin said.

The federally owned facility, comprising 297 acres and employing about 3,500 workers, is located on Seavey Island.

For workers at the Portsmouth shipyard who live in New Hampshire, which has no income tax, the matter is monetary. Maine does tax income, and married workers from New Hampshire find that Maine may use family income to determine their applicable Maine income tax rate.

The court has several options. It could grant the motion to dismiss – allowing Maine to continue to control the property – or could appoint a special master to further investigate the case. The court also could schedule oral arguments to clarify any unresolved issues.