PORTLAND – The U.S. Supreme Court will hear oral arguments on a motion to dismiss a lawsuit arising from a longstanding dispute over whether the Portsmouth Naval Shipyard is in Maine or New Hampshire.

Those who favor changing the border said the court’s order was a victory for New Hampshire in the yearlong legal battle. The court could have dismissed the case outright without hearing oral arguments.

“From our point of view, the longer we remain before a court, it enhances the credibility of the case,” New Hampshire Attorney General Philip McLaughlin said. “In that sense it’s a victory.”

But Paul Stern, a deputy attorney general in Maine, said the court’s decision was anticipated because no one truly expected the court to dismiss the lawsuit without hearing arguments on Maine’s motion.

“It should come as surprising to absolutely no one that the Supreme Court will hear oral arguments on the matter,” he said.

The shipyard is on an island in the middle of the Piscataqua River that separates the two states.

Maine considers its border to be in the middle of the river, which places the shipyard in Kittery, Maine. New Hampshire disagrees, contending the boundary lies along the Maine bank, putting the yard in Portsmouth, N.H.

At stake in the dispute is the $5 million in income-tax revenue that Maine receives annually from 1,300 New Hampshire residents who work at the shipyard. New Hampshire has no income tax.

The issue stirred little interest until Maine adopted a state income tax in 1969. New Hampshire revived the debate by asking the U.S. Supreme Court last spring to settle the issue once and for all.

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

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.

The Supreme Court likely will schedule the hearing for March or April.

The court could end the matter at that point by dismissing the lawsuit, or it could reject the motion and send the dispute to a special master, which could leave the dispute unresolved for years.

A leading spokesman for the workers’ cause is Victor Bourre of Dover, N.H., who said he was thrilled to learn the Supreme Court will hear oral arguments.

“I couldn’t be more excited,” he said. “It’s something I’ve been working on for a long time. It’s like a dream come true for it to get this far.”