A growing group of Republican senators is condemning President Trump’s deal with Russian President Vladimir Putin to import an estimated 2.5 million tons of Russian diesel as a “gift to our adversary and a betrayal of our allies.”Six Republican senators, including former Majority Leader Mitch McConnell (Ky.) and Sen. John Cornyn (R-Texas), signed a bipartisan letter to Trump Tuesday slamming the president’s agreement to buy hundreds of thousands of tons of Russian diesel, despite a 2022 law prohibiting such a purchase without notification to Congress, as a grave mistake.

“The deal you struck with President Vladimir Putin on October 9th will not achieve just ends, nor will it benefit Americans,” they wrote, warning that lifting sanctions on Russia’s diesel exports “only provides Putin with more fuel to finance his war machine and enables Russia to continue its brutal attacks on Ukrainian civilians.”Republican Sens. Alan Armstrong (Okla.), Thom Tillis (N.C.), Lisa Murkowski (Alaska), and Susan Collins (Maine) also signed the letter.

Tillis was the lead Republican signatory. Six Democrats also signed it: Sen. Richard Blumenthal (Conn.), the lead Democratic signatory, and Sens. Jeanne Shaheen (N.H.), Dick Durbin (Ill.), Sheldon Whitehouse (R.I.), Jack Reed (R.I.) and Chris Coons (Del.).

The senators acknowledged that Trump is trying to lower high diesel prices, which reached a record high of $6.53 a gallon last month, but they warned the import of Russian diesel would have little impact on prices in the United States. “Allowing Russian-origin diesel into global markets will offer limited to no relief to Americans. It will have a negligible impact on fuel prices but the revenues the Kremlin will receive from this agreement will undoubtedly finance its desired war of attrition and advance security cooperation with China, Iran and North Korea,” they wrote.

They warned that could ultimately “endanger servicemembers operating overseas in support of Operation Epic Fury,” referring to the military conflict with Iran. The senators circulated the letter a day after Ukrainian President Volodymyr Zelensky said that Russia violated a ceasefire by striking at least six energy targets across Ukraine. Zelensky warned that allowing Russia to collect more revenue from energy sales would fund “its war machine.”

The GOP senators and their Democratic colleagues noted that last month Congress passed and Trump signed the Lindsey O. Graham Sanctioning Russia and Iran Act, which granted Trump authorities to apply economic pressure on Russia and its economic partners, such as China, to enhance the U.S. negotiating position to bring an end to the four-year-old Ukraine war. “For decades, America has stood as the bastion of freedom against tyranny, and American presidents have embodied our nation’s resistance to tyrants. Putin is a tyrant. Withdraw from this agreement, rescind General License No. 135, and execute the authorities provided under the Lindsey O. Graham Sanctioning Russia and Rian Act,” the wrote, referring to the Treasury Department’s general license lifting Russia sanctions.

“Maintain our legacy — and the duty America holds — as leader of the free world,” the wrote. Tuesday’s letter followed a joint statement a bipartisan group of senators, including Collins, Murkowski, Tillis and Sen. John Curtis (R-Utah) released on Sunday pointing out that Congress “explicitly banned Russian energy imports in 2022 to prevent American dollars from financing Putin’s brutal war against Ukraine.”Curtis did not sign the Tuesday letter to Trump.