Sen. John McCainJohn Sidney McCainCongress brings back corrupt, costly, and inequitably earmarks Trump knocks CNN for 'completely false' report Gaetz was denied meeting The Hill's Morning Report - Presented by Tax March - Biden, lawmakers start down a road with infrastructure MORE (R-Ariz.) said Saturday he has concerns about the possibility of Exxon Mobil CEO Rex Tillerson being nominated for secretary of State, given his ties to Russian President Vladimir Putin.

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“I don’t know what Mr. Tillerson’s relationship with Vladimir Putin was, but I’ll tell you it is a matter of concern to me,” McCain said in an interview with Fox News.

NBC News reported earlier on Saturday that President-elect Donald TrumpDonald TrumpFreedom Caucus member condemns GOP group pushing 'Anglo-Saxon political traditions' MyPillow CEO Mike Lindell's new free speech site to ban certain curse words Secret Facebook groups of special operations officers include racist comments, QAnon posts: report MORE had settled on Tillerson to head the State Department, though the transition team has not confirmed the report.

McCain is the first Republican to raise questions about the potential nomination, and Tillerson could fail to make it through the confirmation process if just three GOP senators join a united Democratic opposition.

“I’d have to examine it,” McCain said of the potential nomination. “You want to give the president of the United States the benefit of the doubt because the people have spoken. But Vladimir Putin is a thug, bully and a murderer, and anybody else who describes him as anything else is lying.”

Former top McCain aide Mark Salter predicted that Tillerson would have a hard time getting through the confirmation process.

“Tillerson would sell out NATO for Sakhalin oil and his pal, Vlad. Should be a rough confirmation hearing, and a no vote on the Senate floor,” Salter tweeted Friday.

Tillerson negotiated an energy partnership with Putin in 2011 that the Russian president said would be worth as much as $500 billion. The next year, the Exxon Mobile CEO received the Russian Order of Friendship from the Kremlin, one of the highest honors that Russia bestows on foreigners.

The energy deal was put on hold when the U.S. levied sanctions against Russia for annexing Crimea. Reuters reported earlier this year that Exxon vowed to resume the agreement once sanctions are rolled back, a process Tillerson would be heavily involved in as secretary of State.