The cost of building Virginia-class attack submarines could grow by up to $600 million if Congress signs off on the Navy’s proposal to slip a Virginia from 2014 to 2018. Under heavy pressure to cut budgets, the Navy wants to reduce sub-building expenses in the short term, even at the price of increasing the program’s overall cost. But two powerful legislators, longtime sub-booster Sen. Joe Lieberman of Connecticut (formerly a Democrat but now an independent) and House Armed Services Chairman Buck McKeon (R.-Calif.), are rallying opposition to the delay.

The U.S. Navy, submarine builders, and their allies on Capitol Hill spent years trying to drive down the cost of the world-beating Virginias. The effort included design changes, industrial initiatives, and focused politicking. The goal: to ensure the Navy could afford to buy two Virginias a year for a bit over $4 billion (in 2005 dollars), starting in Fiscal Year 2012.

The “2 for 4 in ’12″ effort succeeded. Last year the Navy requested, and Congress approved, the acquisition of two submarines in 2012 for a combined cost of $4.7 billion. Navy plans at the time anticipated building two attack boats a year through 2022, with the exception of 2018, the only year in which just one Virginia would be purchased. The plan sustained a 48-strong attack submarine force over the long term, according to Navy projections.

But just a few months later, the 2013-2017 Five-Year Defense Plan announced in February bumped one of the two Virginias planned for 2014 back to 2018. The overall number of submarines the Navy intends to purchase in coming decades would not change, only the timing. But even a shift in the schedule can make a big impact on costs, industry officials say.

Nuclear-powered attack submarines are some of America’s most powerful (and expensive) weapons. “Our submarines provide a unique and uniquely American capacity for endurance, mobility and stealth,” a staffer in Lieberman’s office told Breaking Defense, speaking on condition of anonymity.

Lieberman sits on the Senate Armed Services Committee and has long been a strong advocate for expanded submarine production. He and other submarine boosters want the 2014 boat restored. “There is no good reason for this delay,” Lieberman wrote in a February joint statement with two other Connecticut lawmakers. “We are concerned about the impact this change could have on the industrial base.” (A Navy spokesman did not respond to request for comment.)

“We had worked very hard to get the cost of the program down,” said John Holmander, the program manager for the Virginia class at Connecticut-based Electric Boat, which handles all design work on the subs and shares construction work with Virginia’s Newport News Shipbuilding. “Unfortunately,” Holmander said, “moving one of those ships out of Fiscal Year ’14 and moving it to ’18 drives the total cost of the program up by at least $600 million.” That works out to just under $70 million for each of the nine so-called “Block IV” attack subs planned for the period 2014 to 2018.

“Moving a ship by that much really does have that effect,” Holmander went on. “The way you go out and procure materials … when you go to one per year when there been two causes an interruption in the process, and that has inefficiencies that go along with it.”

Responding to mounting pressure from pro-submarine lawmakers, Defense Secretary Leon Panetta said he was “prepared to look at” adding back the delayed sub, provided additional cash could be found elsewhere in the defense budget.

But the Republican-controlled House isn’t waiting for Panetta to scrounge for extra money. The legislative body is set to vote on a proposed budget that adds $25 billion to the White House’s proposed $525 billion defense budget for 2013. The proposed House budget could restore the delayed Virginia.

“What I’ve found is, it’s not over ’til it’s over,” Holmander said.