WASHINGTON—Mitt Romney, doubling down on his contention that President Obama has allowed the Navy to dangerously erode, launched a television ad Thursday in New Hampshire seeking support from voters who depend on jobs with shipyards and other defense industries.

In the ad, titled “Our Navy—New Hampshire,” the former Massachusetts governor makes a patriotic appeal for increased defense spending and blames Obama for pending defense cuts that were also approved by both parties in Congress.

“The state of our Navy—the state of the entire US military—is crucial for America,” the narrator intones. “Our freedom depends on it. But so do many of our jobs—3,600 in New Hampshire alone.”

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The ad also seeks to portray the cuts as characteristic of an administration that has weakened America’s influence in the world: “Does President Obama know how much his defense cuts will hurt us? ... Do they also expose how President Obama views the world and America’s place in it?

By highlighting his repeated calls on the campaign trail for a larger fleet, Romney is trying to sway undecided voters in a state that could prove crucial in deciding who will win the national race.

The size of the fleet was a flashpoint in the third and final debate between Romney and Obama on Monday.

The new ad opens with Romney’s assertion in the debate that “Our Navy is smaller now than at any time since 1917. The Navy said they needed 313 ships to carry out their mission. We’re now at under 285. We’re headed down to the low 200s if we go through [planned defense cuts]. That’s unacceptable to me.”

Obama ridiculed Romney for the simplistic comparison.

“We also have fewer horses and bayonets, because the nature of our military’s changed,” Obama said. “We have these things called aircraft carriers, where planes land on them. We have these ships that go underwater, nuclear submarines.

“And so the question,” Obama added, “is not a game of ‘Battleship,’ where we’re counting ships. It’s what are our capabilities?”

But the shipbuilding industry is vital to New Hampshire and the broader New England economy, said Loren Thompson, a defense expert at Source Associates, a consulting firm.

“Any increase in shipbuilding,” he said, “will be highly advantageous to Bath Iron Works in Maine, Electric Boat in Groton, Connecticut, and makers of naval electronics in Massachusetts and New Hampshire.”

Shipbuilding and defense cuts are also key issues in the tossup state of Virginia.