Sen. John McCain has become the first Republican to express doubts about President-elect Donald Trump’s potential nomination of Rex Tillerson for secretary of state. On Saturday, McCain told Fox News he was weary of Tillerson’s links to Russia. “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,” he said. “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,” he said. McCain’s former aide, Mark Salter, noted on Twitter that Tillerson might not even make it through the confirmation process, writing: “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.” Trump’s transition team has yet to confirm the nomination of the Exxon Mobil CEO for the position, but numerous reports have cited sources close to the matter saying Tillerson has been tapped.