Russian President Vladimir Putin is often invoked as a new ally in a Trump administration. But Republican lawmakers are trying to change U.S. nuclear policy to harden the country’s stance against Russia. (Jure Makovec/AFP/Getty Images)

Donald Trump has sparked a presidential debate about whether and how the United States should use weapons from its nuclear arsenal.

But behind the scenes here in Washington, Republican lawmakers are quietly planning to ramp up development of defensive weapons in a move that could change long-standing U.S. policy and rock global relationships.

It all comes down to one word — “limited” — that currently defines the type of nuclear strike the U.S. guards against by tightly controlling the number of ballistic missiles in the country’s defensive arsenal. It’s a policy focused on threats from places like Iran and North Korea.

But now Congress is planning on removing that word and potentially replacing it with a more “robust” substitute, letting the next president significantly ramp up production, modernization and development of defensive weapons aimed against bigger nuclear powers.

Proponents of the unprecedented change, led by Trent Franks (R-Ariz.) in the House and Ted Cruz (R-Texas) in the Senate, believe it is necessary to help the U.S. respond to increased threats from nations like Russia and China.

Russia has become a flashpoint of political debate on the campaign trail this year, with critics alleging that Trump and has campaign are too close to Russian President Vladimir Putin and Trump calling on Russia to hack Hillary Clinton’s email server to find 30,000 emails that were previously deleted.

[Inside Trump’s financial ties to Russia]

But most Republicans are far more alarmed by Russia, pointing to Putin as a “gangster,” in the word of Sen. Marco Rubio (Fla.), who  is wreaking havoc in Syria and Ukraine, and count Russia as a key reason the U.S. must build up its military arsenal and fortify its ballistic defenses.

“During his administration, Trump will be friendly with Putin. I think it would be great,” the GOP nominee recently told CNBC.

But many experts warn the change will broadcast a dangerous geopolitical message to U.S. rivals, who may use it to justify building up offensive ballistic missile capacity, potentially putting nuclear arms control treaties in jeopardy.

“The calculus of the nuclear arms race is that if one side builds up defenses, the other side builds up offensive systems to overwhelm those defenses,” said Joe Cirincione, a nuclear security expert and president of the Ploughshares Fund. “We are on the verge of repeating the Cold War errors we escaped.”

The “limited” language is being contemplated as part of the annual defense policy bill, the final draft of which is being negotiated. Yet there is consensus between the House and Senate that the existing standard should change: the Senate-passed bill strikes the word “limited,” while the House-passed bill states U.S. policy should be to “maintain and improve a robust, layered missile defense system” to guard against “the developing and increasingly complex ballistic missile threat.”

What that means in practice, however, is a subject of considerable dispute — partly because “limited” has no precise definition.

Congress adopted the “limited” standard as part of the National Missile Defense Act of 1999, at a time when U.S. ballistic missile defense was already constrained by the Anti-Ballistic Missile (ABM) Treaty, which is no longer in force. Since then, U.S. relations with rival nations like Russia and China have worsened.

In the present environment, some argue the word “limited” reassures rivals that the U.S. is exercising restraint in its nuclear defense planning. They believe erasing “limited” could trigger a new arms race even if the U.S. never rolls a single additional missile interceptor off the line.

But others forcefully dispute that suggestion carries any inherent dangers, arguing it gives the United States room to plan for growing threats without committing it to any tangible changes.

A new legal standard for acceptable ballistic missile defense is not in itself a cue to defense contractors to churn out missile interceptors bound for Alaska and Europe or a promise the U.S. will soon launch technologically advanced defense systems into space. Such moves depend on the pace of research and development, congressional appropriations, and presidential priorities.

Because of those practical limitationsexperts like Thomas Karako, director of the Center for Strategic and International Studies’ Missile Defense Project, call Congress’ legal changes “aesthetic” and fears about their impact overblown.

“There’s nothing that we’re doing that comes even close to upsetting the strategic balance,” Karako said, adding that even under the “limited” standard, there is nothing “in the legislative language right now that prevents us from doing whatever it is that we want to do.”

But other experts warn the danger is not about anything tangible, but in the political signal a new policy sends.

“Russia never really bought the argument that U.S. missile defense is against Iran and North Korea…so if you remove the word ‘limited,’ then basically the Russians will say, ‘Aha, you see, we were right all along, it was all about us,’” said Nikolai Sokov, a nuclear security expert at the James Martin Center for Nonproliferation Studies. “It will be seen as ‘They finally stopped the pretense.’ That’s not good, in my view.”

Russia’s convictions about U.S. missile defense were on display earlier this year, when Putin said Moscow would respond to the U.S. placing defensive missile systems in Romania. Sokov said abandoning the “limited” standard would likely encourage Moscow and Beijing to take more retaliatory steps, including developing nuclear weapons aimed at piercing enhanced U.S. defenses.

Sokov and Cirincione both argued that comprehensive missile defense — which the U.S. has pursued since the Reagan administration — is an unattainable goal.

But advocates of strengthening the nation’s nuclear defensive capacities point out that Russia and China are already modernizing their nukes, leaving the U.S. no choice but to step up its response.

“You gotta start somewhere,” said Trey Obering, an executive vice president at Booz Allen Hamilton who directed the Pentagon’s Missile Defense Agency (MDA). He stressed the government should invest more in the MDA to come up with next-generation defenses, even if it takes several years to get them online.

“We’re seeing a reemerging, belligerent Russia and a very aggressive China, and we can’t kind of stick our heads in the sand and have this make-believe world where they don’t exist,” Obering said. “If the word ‘limited’ is getting in the way of us being able to defend against all threats to our nation, then it needs to be removed.”

But words aren’t the only barrier: developing nuclear defenses is no cheap and easy task. Lawmakers disagree about how to allocate limited funds to modernize the defensive systems that currently exist. And it takes more resources to defend against a ballistic missile than to launch one.

“What we have now is really expensive,” said Richard Weitz, a security and arms control expert at the Hudson Institute. “In missile defense that’s always been the problem: you have to launch multiple times and maybe you’ll knock out an incoming missile, but it’s so easy for the other side to throw lots and lots more missiles.”

With limited budgets, few experts think mass producing current missile interceptors is the answer. Today, U.S. missile defenses relies on mechanisms like the Terminal High Altitude Area Defense, or THAAD, systems — kept on a base in Cruz’s home state of Texas — Aegis missile systems and Patriot missile systems, which are manufactured by companies like Lockheed Martin and Raytheon, whose missile systems company is based in Tucson, a city Franks represents.

But nobody knows how much developing more advanced systems will cost. Another part of the House’s defense policy bill commissions a study to review the capacity of ballistic missile defense, with instructions to pay attention to researching space-based systems.

Such strategic proposals have not encountered serious opposition in Congress, despite arguments elsewhere about funding nuclear modernization.

“Everyone wants to be safe, so it’s a good public point — how could you oppose that?” said Stephen Young, a nuclear expert with the Union of Concerned Scientists. “But the money’s not there to do what the policy calls for.”

Ultimately, the next president will be most responsible for implementing any policy changes. And neither of the two candidates seem as intently focused on ratcheting up missile defense as Congress. 

Hillary Clinton “may be more open to taking missile defense action that the Russians or the Chinese would object to than the Obama administration,” Weitz said. “Trump, from what we know with his statements, he thinks it’s all a waste of money, so he would just ignore Congress.”