WASHINGTON — More than a quarter of the Senate sent a letter to President Joe Biden on Thursday threatening to tank a $20 billion arms sale that includes 40 Lockheed Martin Block 70 F-16 fighter jets and upgrades to Turkey’s current fleet so long as Ankara continues to block Sweden and Finland from joining NATO.

The letter comes days after another senator on the Foreign Relations Committee floated the prospect of sanctions on Turkey should it continue blocking the two Nordic countries from joining the alliance.

“Once the NATO accession protocols are ratified by Türkiye, Congress can consider the sale of F16 fighter jets,” Sens. Jeanne Shaheen, D-N.H., and Thom Tillis, R-N.C., wrote in the letter to Biden, signed by a bipartisan group of 25 other senators. “A failure to do so, however, would call into question this pending sale.”

The signatories included multiple senators who sit on the Armed Services panel and the Foreign Relations Committee, which has jurisdiction over arms sales. The Senate’s No. 2 Democrat, Dick Durbin of Illinois, also signed the letter.

The senators argued Turkey is violating its commitments under a trilateral agreement it signed last year with Finland and Sweden. Under that agreement, the two countries would take action on the outlawed Kurdistan Worker’s Party, or PKK, and potentially extradite certain individuals in exchange for Turkish ratification of their NATO membership applications.

“Sweden and Finland have moved forward to address the issues in their memorandum of understanding, so I hope that would trigger ratification,” Shaheen told Defense News.

Sweden in particular landed in Turkish President Recep Tayyip Erdogan’s crosshairs last month after a far-right Danish politician travelled to Stockholm to burn a Qur’an near the Turkish embassy. The incident prompted Turkey to cancel a visit to Ankara from Sweden’s Defense Minister Pal Jonson, where he had hoped to discuss its NATO bid.

Still, the senators made it a point to note Turkey “has proven to be a valuable NATO ally as Russia’s illegal invasion of Ukraine continues,” and praised it for its “commitment to implementing the United Nations-brokered grain deal which has allowed Ukraine to export grain and avert a global food crisis.”

Looking to sanctions

At least two Senate opponents of the sale did not sign onto the Shaheen letter and favor an even harder line on Turkey.

“The F-16 issue is far greater than just NATO ascension, although that is part of it,” Menendez told Defense News last month. “But just doing that, it doesn’t solve the problem.”

Menendez’s home state of New Jersey boasts large Greek-American and Armenian-American populations, making Turkey particularly unpopular among some of his constituents.

And on Tuesday, Senate Foreign Relations Committee member Chris Van Hollen, D-Md., raised the prospect of sanctioning Turkey for refusing to ratify Sweden and Finland’s NATO membership.

“We need to be working in coordination with our [European Union] partners and considering potentially different kinds of sanctions if Erdogan continues to block entry of Finland and Sweden,” Van Hollen said at an event hosted by Al-Monitor.

“The Turkish economy is not in great shape,” he continued. “It’s one of the reasons that polling in Turkey shows that Erdogan is not popular. So, I do think there have to be some consequences to this kind of conduct.”

The Trump administration in 2020 imposed narrow penalties on Turkey’s military procurement agency over its purchase of the S-400 missile defense system as required under a Russia sanctions law Congress passed in 2017. The S-400 sale also prompted the U.S. to expel Turkey from its F-35 co-production program amid fears the Russian system’s powerful radar could be used to spy on the stealth fighter jet.

In addition to sanctions for blocking Swedish and Finnish NATO entry, Van Hollen also raised the prospect of sanctions should Erdogan make good on his increasing threats to attack U.S.-backed Kurdish forces in northeastern Syria ahead of Turkish presidential elections in May.

“This is an area where I think there have to be consequences in terms of joint U.S.-European actions in the form of sanctions,” Van Hollen said at the Al-Monitor event. “The Europeans have in the past also supported and continue to support the U.S. position here.”

Elham Ahmad, the president of the Kurdish-dominated administration in northeast Syria, will visit Washington next week with the aim of thwarting another Turkish incursion, particularly against Kobane, a Kurdish-majority city on the Turkish-Syrian border. She has previously lobbied Congress against the F-16 sale, noting Turkey has used its existing F-16 fleet to target civilian infrastructure in northeast Syria.

Sen. Tim Kaine, D-Va., who signed onto the F-16 letter, told Defense News “Turkey’s incursions now into Kurdistan and northern Iraq are very troublesome.”

“We’ve always had a challenge with them and the Kurds in northern Syria, but the number of challenges seem to be piling up,” he continued.