President Trump's team must deny a request from Exxon Mobil to proceed with a major energy deal in Russia, according to a top Democrat.

"This is a no-brainer," Rep. Eliot Engel, the ranking member on the House Foreign Affairs Committee, said Thursday. "The administration's answer to Exxon Mobil should be an unequivocal no."

Trump has maintained the economic sanctions imposed by then-President Barack Obama in response to Russia's aggression against Ukraine, despite initial fears he would be inclined to lift them. Exxon Mobil, however, has reportedly applied for a waiver from the sanctions so it can implement a deal struck with a state-run energy company that Russian President Vladimir Putin estimated could be worth $500 billion.

"Not only has Vladimir Putin attacked our democratic process, he is bolstering the Assad regime, waging an illegal war in Europe, and suppressing the rights of the Russian people," Engel said. "The last thing the United States should be doing is clearing the way for lucrative new business opportunities for Putin and his cronies."

Democrats say a waiver for Exxon Mobil would be just the kind of exception that undermines the effectiveness of sanctions regimes. It could cause other leaders to weaken their own sanctions, making the same kind of argument against the Ukraine sanctions that Secretary of State Rex Tillerson made in 2014 when he was CEO of Exxon Mobil.

"We do not support sanctions, generally, because we don't find them to be effective unless they are very well implemented comprehensively and that's a very hard thing to do," Tillerson told his shareholders in May of 2014.

Tillerson has recused himself from government business pertaining to his former company, but Engel argued that their past relationship is an additional reason not to grant the waiver.

"Granting Exxon Mobil a waiver of Russia sanctions would play into Vladimir Putin's hands and deepen concerns about the Trump administration's cozy ties with Moscow," Engel said. "[T]he fact that the Secretary of State's longtime private-sector home is asking for this special treatment casts a cloud of ethical concerns over the entire landscape."