WASHINGTON ― Former Exxon Mobil chief executive Rex Tillerson attempted to alleviate concerns about his past ties with Russia during a series of meetings with senators from both parties on Wednesday.

During his 10 years in charge at Exxon Mobil, Tillerson, who has been tapped to serve as secretary of state, developed close ties with high-ranking Russian officials, including President Vladimir Putin. In 2012, the Kremlin awarded him the country’s Order of Friendship. He has been an outspoken critic of sanctions, one of the main tools used by the Obama administration to combat Russia’s aggression in Ukraine and interference in the U.S. election. Tillerson argued in 2014 that sanctions are difficult to execute effectively ― but sanctions targeting Russia have also cost Exxon Mobil over $1 billion.

For those reasons, lawmakers are skeptical that Tillerson is the right person to steer America’s diplomatic relations at a time when U.S.-Russia relations are nearing a post-Cold War low point. Many are wary of President-elect Donald Trump, who has called Putin a strong leader, has suggested ceding Washington’s role in Syria to Moscow and has disavowed U.S. intelligence assessments that Russia hacked the Democratic National Committee in an effort to interfere with the 2016 election. For those hoping Trump’s close advisers might redirect his Russia policy, Tillerson’s nomination was a disappointment.

“Russia is not a friend of the United States,” Sen. Ben Cardin (D-Md.) said after meeting with Tillerson on Wednesday. “Obviously, [Tillerson] did business with Russia, he was able to get things done there, and those relationships will be subject to questioning during the confirmation.”

Cardin, the ranking member of the Senate Foreign Relations Committee, said he has not yet decided whether he will vote to confirm Tillerson, who will testify before the full committee next week. Lawmakers received Tillerson’s financial disclosure paperwork on Tuesday, Cardin said. Democrats are also pushing for copies of his tax returns from the past three years, a request that committee Chairman Bob Corker (R-Tenn.) has said is unnecessary.

Tillerson needs a simple majority in the Senate to be confirmed, meaning that Republicans, with 52 seats, could cement his nomination without any bipartisan support. Typically the more hawkish party, several Republicans have shied away from condemning Moscow or Tillerson’s ties to Russian oligarchs, as doing so would clash with the policy of the president-elect. Senate Majority Leader Mitch McConnell (R-Ky.) and Sen. Tom Cotton (R-Ark.) ― who has hammered the Obama administration for not taking a more aggressive posture with Russia ― have both met with Tillerson and said they will support him.

But Tillerson could still have trouble securing the support of a handful of members from his own party. GOP Sens. John McCain (Ariz.), Lindsey Graham (S.C.) and Marco Rubio (Fla.) have all raised concern about the former Exxon Mobil chief’s ties with Putin.

McCain, who was scheduled to speak with Tillerson on Wednesday afternoon, stopped just short of vowing a “no” vote. Asked if he could back Tillerson, McCain told reporters ahead of their meeting, “Sure ― there’s also a realistic scenario that pigs fly.” (McCain’s spokeswoman Rachael Dean later clarified that her boss was joking. “He has repeatedly said that he has concerns regarding Mr. Tillerson’s nomination, but has made no decision,” she wrote in an email.)

Graham and Rubio, a member of the Senate Foreign Relations Committee, have not yet met with Tillerson.

Democrats who have met with him were tight-lipped about their discussions, but appear cautiously optimistic about the potential secretary of state, whom they may be powerless to block.

“I was encouraged about his clarity around the difference in the role between being CEO of an oil company and secretary of state,” Sen. Chris Coons (D-Del.) said after his meeting with Tillerson, which lasted nearly an hour longer than originally scheduled. During their conversation, said Coons, they talked about America’s core interests in the world and its role in promoting press freedom, human rights and democracy.

Tillerson told Cardin that he supported the Paris Agreement, an international accord aimed at curbing greenhouse gas emissions ― a problem Trump has suggested is a hoax crafted by the Chinese. “[Tillerson] stressed for me his background in science and that he was a believer in science,” Cardin said. “That was also encouraging.”

In some ways, Democrats have more to lose than Republicans if Tillerson fails to make it through the confirmation process. Trump is likely to shift U.S. policy in a direction that is more favorable to Moscow, regardless of who serves as the nation’s top diplomat. Tillerson, according to lawmakers who have spoken with him, appears to at least be knowledgeable and rational. For many Democrats, Tillerson’s nomination is easier to stomach than some of the previously rumored alternatives, such as John Bolton or Rudy Giuliani.

This article has been updated to include comment from Dean.