Trump shocked lawmakers when he announced on social media Friday that he struck a deal with Russian President Vladimir Putin in a phone call. He said Russia will immediately supply more than 300,000 tons of diesel to the American and global market, followed by an additional 500,000 tons in November and 1 million tons “immediately thereafter.” Russia will deliver an additional 3 million tons “within a short period of time” after that, he wrote.

Four Republican senators and two Democratic senators signed onto the statement released Sunday in protesting the deal. They said the administration failed to consult with Congress or submit any explanation to justify its decision as required by statute.

“The United States should not provide Putin with a financial lifeline while Russia continues to kill Ukrainian civilians. The Administration must immediately reverse course,” they wrote.

The Republican senators who signed onto the statement are Sens. Susan Collins of Maine, Lisa Murkowski of Alaska, Thom Tillis of North Carolina and John Curtis of Utah. The Democratic senators are Sens. Jeanne Shaheen of New Hampshire and Richard Blumenthal of Connecticut.

Tillis, in an interview that aired Sunday on NBC’s “Meet the Press,” said there’s a legitimate case for saying the president is not following the law if the administration goes through with its diesel purchase plan.

“We were very clear that Russia is the aggressor. Russia is the one who can end this war by backing out of territory that they occupied,” Tillis said.

Trump’s announcement came just a few weeks after Congress passed legislation that directed Trump to impose substantial tariffs on the top importers of Russian oil or natural gas, with an exception for countries that import less than 15% of Russia’s natural gas exports and have taken significant steps to reduce those imports.

The congressional effort was aimed at cutting the flow of money Russia receives from oil and gas sales to finance its war against Ukraine.

The lawmakers also noted that Congress explicitly banned Russia energy imports in 2022 to prevent the United States from financing the war.

“Congress was clear: the U.S. should not import oil from Russia until it ends its aggression against Ukraine and ceases threats against our NATO Allies,” the six lawmakers wrote in their statement. “President Trump’s decision to allow Russian diesel imports to the U.S. runs directly counter to that law.”

Sen. John Barrasso, R-Wyo., said Trump is focused on reducing the cost of energy when asked whether the president’s action would be in violation of U.S. law.

“The president is balancing and has to make tough decisions,” Barrasso said on CBS’s “Face the Nation.”

He also encouraged Trump to enforce the sanctions law that Congress passed last month.

“This is just another point of leverage for the president to use against Putin in an effort to get a long-term solution in that part of the world,” Barrasso said.

Trump has recently sought to blame record U.S. diesel prices on Ukraine rather than the U.S.-led war in Iran, which has elevated world energy prices.