Sen. Christopher Coons Christopher (Chris) Andrew CoonsMurkowski: Supreme Court nominee should not be taken up before election Battle lines drawn on precedent in Supreme Court fight Sunday shows - Ruth Bader Ginsburg's death dominates MORE (D-Del.) expressed concern on Sunday that President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE could offer Russian President Vladimir Putin a number of concessions when the two leaders meet in Helsinki on Monday.

"There’s a menu of things to be concerned about — that he might withdraw American troops from Syria, that he might cancel military exercises with our regional allies, that he might recognize Russia’s annexation of Crimea," Coons said in an interview on "Fox News Sunday."

Coons, a member of the Senate Foreign Relations Committee, also sought to remind Trump not to treat his upcoming summit with Putin like he would a meeting with a U.S. ally.

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"He’s not meeting with a competitor, he’s not meet with a potential friend," Coons said. "He’s meeting with an adversary."

Trump said in Brussels on Thursday that Putin was a "competitor," rather than an adversary, adding that the upcoming summit was not "a question of friend or enemy."

"He’s not my enemy," Trump said.

The Delaware Democrat's comments came as Trump prepares to sit down with Putin on Monday for a highly anticipated summit.

The U.S. president has repeatedly expressed a desire to thaw relations with Moscow. But that task has been complicated by the U.S. intelligence community's determination that Russia interfered in the 2016 presidential election and is poised to do the same in the 2018 midterms.

It's also complicated by the ongoing special counsel investigation into whether the Trump campaign coordinated with the Russians in 2016 to boost the real estate mogul's presidential bid.

Trump has dismissed the probe as a "witch hunt."