GROTON, Conn. — Here in the Submarine Capital of the World, where the business of undersea warfare employs nearly 20,000 people, no one dismisses Mitt Romney’s plan to build three Virginia-class nuclear attack submarines a year instead of the two built each year under President Obama.

But with each submarine costing more than $2 billion, there is skepticism about how to pay for them, even from Representative Joe Courtney, a Connecticut Democrat who in 2007 successfully pushed to have the Navy build two submarines a year instead of one and has been known ever since as “Two Subs Joe.”

“You can definitely make a case” to build three submarines a year, said Mr. Courtney, whose district is home to the Naval Submarine Base New London and to General Dynamics Electric Boat, both in Groton. Nonetheless, the Romney campaign’s math for how to do it, he argued, “is not realistic at all.”

Whoever is right, building more military ships is at the heart of Mr. Romney’s plan to gradually increase military spending to 4 percent of the nation’s gross domestic product, a major increase from the Obama administration’s budget.