(Bloomberg) — The White House budget office and the Navy are rushing to find an extra $600 million to buy a second littoral combat ship after including only one in the budget that President Donald Trump proposed this week.

Allison Stiller, the Navy’s acting weapons buyer, disclosed the unusual budget maneuver Wednesday at a House Armed Services seapower panel whose members are sympathetic to adding more of the vessels made in competing versions by Lockheed Martin Corp. and Austal Ltd. A second ship would guarantee that each company will get to build one, keeping both of their shipyards running.

“The administration recognizes the criticality of our industrial base and supports funding a second LCS” in the fiscal 2018 budget, Stiller said. Referring to the Office of Management and Budget, she told reporters that adding the ship is “OMB’s prerogative.”

The littoral combat ship, designed for missions in shallow coastal waters, has drawn criticism from Pentagon testing officials and some lawmakers over its reliability and its vulnerability in combat. But the Navy supports the ship, which would help Trump reach his pledge for a 350-ship Navy, up from today’s fleet of 275 vessels that can be deployed.

The budget office “will be publishing a budget errata that adds another LCS to the FY 2018 budget request,” Lieutenant Colonel Eric Badger, a Pentagon spokesman, said in a statement. “The total request is two LCS ships. The Navy will identify” where the funds will come from, he said.

“The printed budget request included one LCS because the facts and need for a second came to us so late in the process,” OMB spokeswoman Meghan Burris said. “We understand that Congress is moving quickly to put together FY18 bills, and wanted to get the change in front of them as quickly as possible.”

Burris said “we are still working through the funding, and the official communication to Congress will be delivered upon completion” of that process.

The Trump budget proposal sent to Congress on Tuesday requested $636 million for one littoral combat ship. Instead of a second, the Pentagon and Navy had agreed to go for two DDG-51 Flight III destroyers rather than one.

Pentagon officials were surprised when OMB decided to add a second littoral combat ship too.

Hours before Stiller’s testimony, Acting Navy Secretary Sean Stackley told the Senate defense appropriations panel that the budget sought just one littoral ship because “in2018, budget-wise, we don’t have the capacity to grow in terms of procurement and modernization.”

The Navy says it’s important to maintain the workforces of both Lockheed in Wisconsin and Austal in Alabama until the Navy is ready to pick one of the contractors in mid-2020 to build a better-armed frigate as the successor to the planned fleet of as many as 30 littoral combat ships.

“We want to keep the LCS and the frigate heel-to-toe as best as possible, so that we have a healthy industrial base to compete for that future frigate program,” Stackley said.

To OMB, the decision isn’t about the specifics of the LCS program but about “not wanting either Wisconsin or Alabama — where the LCS’s are built and both Trump states — to suffer,” Mark Cancian, a budget analyst with the Center for Strategic and International Studies who handled defense issues when he worked for the budget office, said in an email.

The Government Accountability Office said last month that Congress should consider delaying the Navy’s expected request for as much as $9 billion to start work on as many as 12 frigates. The GAO said too many unanswered questions remained about the new vessel’s cost and capabilities.

Bloomberg News by Tony Capaccio