After the Congressional Budget Office (CBO) estimated that withdrawing from the last remaining nuclear arms control treaty could potentially result in an arms buildup that costs the U.S. anywhere between $100 million and several hundred billion dollars, the Democratic lawmakers who ordered up the report urged President Donald Trump to renew the treaty.

“The Trump administration’s unwillingness to continue the decades of strategic arms control by failing to extend the New START Treaty is driving the United States toward a dangerous arms race, which we cannot afford,” Rep. Adam Smith (D-Wash.) and Sen. Bob Menendez (D-N.J.) wrote Wednesday in a joint statement. “While this report only begins to account for the costs of the Administration’s preposterous claims that we can ‘spend the adversary into oblivion,’ it is further proof of why New START is essential to U.S. and international security.”

Smith is chair of the House Armed Services Committee. Menendez is the ranking member of the Senate Foreign Relations Committee.

New START, ratified in 2011 during the Barack Obama administration, limits the U.S. and Russia to deploying 1,550 strategic nuclear weapons on a mixture of 700 intercontinental ballistic missiles, submarine-launched ballistic missiles, and heavy bombers. Previous bilateral nuclear weapons treaties with Russia allowed for many more warheads than that.

The CBO estimated flexing up to the limits allowed under 1990s era START I treaty, a maximum, 6,000 warheads on 1,600 strategic delivery vehicles, would cost the Pentagon a one-time charge of $410 billion to $440 billion, plus additional annual upkeep costs ranging from about $24 billion to $28 billion. That was the most expensive option identified in the report.

The cheapest option in the report would be to expand the arsenal into SALT II territory — 3,000 warheads — by putting multiple warheads on all current and currently planned intercontinental ballistic missiles, and submarine-launched ballistic missiles. That’s doable for a one-time Pentagon fee of about $100 million, according to the CBO. SALT II was never ratified in the U.S., but Washington and Moscow observed its limits from the late 1970s to the mid 1980s. 

New START, meanwhile, will expire in February unless the U.S. and Russian presidents agree to extend it. The treaty as written allows for a five-year extension, though some speculate the parties could work out a shorter-term extension.

The Trump administration prefers to replace New START with a trialteral agreement that also curbs China’s nuclear arsenal. In a press conference last week, after returning from New START negotiations in Vienna with Sergei Ryabkov, the deputy Russian foreign minister, Marshall Billingslea, Trump’s special presidential envoy for arms control, accused China of a “secretive crash nuclear buildup.”

China has steadfastly refused to join nuclear arms control talks, and Ryabkov, after the meeting with Billingslea, told the privately owned Russian news service Interfax that Russia is not desperate to extend New START, and that future talks with other nuclear-armed powers, including China, should not influence the current treaty negotiations between Washington and Moscow.

In Washington, there is bipartisan support for extending New START, and bi-partisan support for future talks geared at a follow-up agreement that could cover types of nukes the current pact does not. Among those are Russia’s tactical nuclear weapons — relatively smaller nukes designed to win a battle, rather than wipe out an adversary’s capability to wage war.

Smith and Menendez sidled up to that idea in their statement.

“If the United States lets the New START Treaty expire, Russia, which is already ahead of America’s nuclear modernization program, would use a U.S. exit from the New START Treaty to quickly expand its arsenal without any legal constraints for the first time in 50 years,” the two wrote Wednesday.