Sen. Marco Rubio had been skeptical of President-elect Donald Trump's pick for secretary of state, Rex Tillerson, since just after his nomination was announced.
Rubio's concerns, mainly over his connections to Russian President Vladimir Putin, were put on public view as he questioned the ExxonMobil chief over whether Tillerson believes Putin is a war criminal.
"I would not use that term," replied Tillerson.
Tillerson's response was not satisfactory to Rubio, who later told reporters that he was troubled by some of his answers.
"I think it's important, that if you stand for moral clarity, that you be clear," said Rubio.
Rubio is not the only Republican senator to express concerns about Tillerson over his past friendliness with Putin. But he is the only skeptical Republican, so far, who has the power to derail Tillerson's nomination before it reaches the Senate floor.
The Florida senator sits on the upper chamber's 19-member Foreign Relations Committee, before which Tillerson's nomination hearing took place on Wednesday. If all nine Democrats on the panel vote against Tillerson, they would need only one of the committee's 10 Republicans to join them to prevent the ExxonMobil CEO's nomination from moving to the full Senate for a vote.
Even if Rubio joins Democrats on the committee and votes against Tillerson, the Senate could still vote on his nomination. However, according to the Congressional Research Service, it's rare for the full Senate to consider a candidate if not approved by the committee.
Rubio has made his position clear since last month, echoing the remarks he made to reporters Wednesday.
"The next Secretary of State must be someone who views the world with moral clarity, is free of potential conflicts of interest, has a clear sense of America's interests and will be a forceful advocate for America's foreign policy goals," Rubio said.
Rubio was referring to the deals Tillerson's multinational oil and gas corporation has made with the Russian government, which helped him earn Russia's 2013 Order of Friendship, one of the highest honors a foreigner can receive there.
Other Republicans on the Foreign Relations Committee offered their support for Tillerson following his hearing. Sen. Jeff Flake (R-Arizona) released a statement praising Tillerson's hearing responses.
"I've been impressed by the answers Mr. Tillerson has given me both in our private conversations and at today's hearing," said Flake. "I believe Mr. Tillerson has a keen ability to recognize America's strategic interests and the skills to advance those interests as Secretary of State."
However, once Tillerson's nomination reaches the floor, he faces skepticism from other Republicans. During an appearance on MSNBC's Morning Joe, Sen. Lindsey Graham (R-South Carolina) said he supported Rubio's aggressive questioning and remained skeptical about Tillerson's foreign policy positions.
"I thought Marco did a really good job," said Graham. "Mr. Tillerson is a very accomplished guy, but when it comes to Russia, I want more clarity. I think war crimes were committed under Putin's control and regime. And, you know, we'll see if he can clean up his answers. But it's not about his qualifications as much as it is about how he views Russia and we'll see what happens."
ABC News' Ben Siegel contributed to this report.
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