WASHINGTON — Top U.S. lawmakers are threatening to pass legislation that would bar NATO ally Turkey from buying the F-35 fighter jet, and sanction the country if it buys the Russian S-400 air defense system, ratcheting pressure via a New York Times op-ed on Tuesday.

If Turkey accepts the S-400, “no F-35s will ever reach Turkish soil. And Turkish participation in the F-35 program, including manufacturing parts, repairing and servicing the fighters, will be terminated, taking Turkish companies out of the manufacturing and supply chain for the program,” wrote the bipartisan leaders of the Senate Armed Services Committee and Senate Foreign Relations Committee.

“We are committed to taking all necessary legislative action to ensure this is the case. Turkey is an important partner in the F-35 program, but it is not irreplaceable,” the lawmakers added.

Here’s how F-35 technology would be compromised if Turkey also had the S-400 anti-aircraft system The U.S. Air Force was able to bomb Libya with impunity after knocking out its relic of an Integrated Air Defense System, but what if the country had actually maintained a competent IADS network?

The letter targets Turkish President Recep Tayyip Erdogan’s decision-making calculus, emphasizing economic costs if Turkey loses co-production of the Lockheed Martin-made F-35 and geopolitical costs if Erdogan steps toward Moscow and away from NATO.

The letter is also a signal to Erdogan that he cannot rely on personal diplomatic efforts by U.S. President Donald Trump, as Congress will stand in the way.

Turkey’s minister of foreign affairs, Mevlüt Çavuşoğlu, last week called the S-400 “a done deal” and claimed that Trump has opened the door to keeping F-35 sales to Turkey going. The comments came as the U.S. has stopped delivery of F-35 parts to Turkey, capping months of warnings over the S-400.

SASC Chairman Jim Inhofe, R-Okla., and ranking member Jack Reed, D-R.I., signed the op-ed with Senate Foreign Relations Committee Chairman Jim Risch, R-Idaho, and ranking member Bob Menendez, D-N.J.

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F-35A fighter jets from the 34th Fighter Squadron at Hill Air Force Base, Utah, land at Royal Air Force Lakenheath, England. (Tech. Sgt. Matthew Plew/U.S. Air Force)

If there has ever been a public letter between these lawmakers, it’s rare, which underlines the high stakes for the U.S., which has been lobbying Ankara to reverse course and offered the American-made Patriot air defense system as an alternative to the S-400 since 2012.

The letter acknowledges that U.S. air dominance hinges on the F-35 — “the world’s largest fifth-generation fighter aircraft program,” “with more than a trillion dollars in investment from a dozen international partners, including Turkey” — but the jet’s integration with the S-400 “could enable the Russian military to figure out how the F-35 operates."

“With the S-400 scheduled to arrive in Turkey in July and the F-35s scheduled to arrive in November, it is time for President Erdogan to choose,” the lawmakers wrote. “It is our hope he will choose to abandon the S-400, defend Turkish skies with the Patriot system and save the F-35 arrangement.”

If Erdogan accepts delivery of the S-400, “Turkey will be sanctioned as required by United States law under the Countering America’s Adversaries Through Sanctions Act. Sanctions will hit Turkey’s economy hard — rattling international markets, scaring away foreign direct investment and crippling Turkey’s aerospace and defense industry,” the lawmakers warn.

Among the “severe consequences” if Turkey abandons (or is forced to abandon) the F-35 program, its $1.25 billion-plus investment “will be squandered” and “it will not receive the more than 100 F-35s it planned to purchase, and it will be forced to settle for a less-capable fighter aircraft that will not arrive for many years,” according to the senators.

“Turkish companies that produce parts for the F-35 will see their orders dry up completely. Its F-35 engine maintenance, repair, overhaul, and upgrade facility will see all its work go to other facilities in Europe,” they wrote. “President Erdogan’s hope to make the Turkish defense industry a key pillar of economic growth for the future will be dashed.”

They commit to “do all we can to assist Turkey” if Erdogan walks away from the S-400 and if Russian President Vladimir Putin retaliates in economic sectors where Ankara relies on Moscow. Putin, they say, “is trying to divide Turkey from the West with the S-400s."