“We support the vision of the Australia-UK-US (Aukus) partnership and its potential to change the strategic landscape in the Indo-Pacific. The Aukus agreement is vitally important, but we must simultaneously protect US national security,” the lawmakers said in a letter.

They said the plan to sell three attack submarines to Australia would “unacceptably weaken” the US fleet without a clear plan to replace them.

The letter was led by Senators Roger Wicker, the top Republican on the Senate Armed Services Committee, and Susan Collins, the top Republican on the Senate Appropriations Committee.

They said Biden should make an “Aukus-specific” spending request along with a multi-year plan to increase US submarine production to a minimum of 2.5 Virginia-class attack submarines per year, compared with the 1.2 such submarines currently being produced.

The Virginia class submarines were designed by General Dynamics Electric Boat and Huntington Ingalls Industries.

The multi-stage Aukus project announced in March is planned to culminate in the late 2030s and early 2040s with British and Australian production and operation of a new submarine class – SSN-Aukus – and include “cutting edge” US technologies.

Before that, in the early 2030s, the US is supposed to sell Australia three US Virginia-class nuclear-powered submarines, with an option for Australia to buy two more.

Big questions remain, however, over issues including US curbs on the extensive technology sharing needed and how long it will take to deliver the Virginia-class submarines, given limited US production capacity, even as the perceived threat posed by China that inspired the project mounts.