Rubio -- who refused to say if he would vote for Tillerson after a day of testimony -- pressed the former ExxonMobil CEO hard Wednesday on a slew of human rights questions, pushing him to denounce the behavior of Russia, China, Saudi Arabia and the Philippines and expressing deep disappointment when the Texan refused to do so.
Rubio's vote could make the difference in Tillerson's progress, as Republicans have only a one-vote margin on the Senate Foreign Affairs Committee and Democrats may not support him. But it is possible, should Tillerson stall in committee, for the Senate to bypass the panel and bring the nomination to the full Senate for a vote.
"This is a very important decision and I recognize the partisan split on the committee and what it means," Rubio told reporters after the hearing. "I'm prepared to do what's right."
The Florida senator -- who lost to Trump in the GOP presidential primary and had heated personal exchanges with him during the campaign -- told Tillerson that "moral clarity" is crucial to US foreign policy and expressed disbelief that Tillerson wouldn't deliver judgments on countries' alleged human rights abuses when information is widely available.
"The position you've been nominated to is, in my opinion, the second-most important position of the US government, with all due respect to the vice president," Rubio said.
When the world sees that "the United States is not prepared to stand up and say, yes, Vladimir Putin is a war criminal, Saudi Arabia violates human rights, it demoralizes these people all over the world," Rubio said.
Rubio wasn't alone on the fence. Sen. Lindsey Graham, a South Carolina Republican, said he is also undecided about Trump's choice.
"I think probably what happened today is he is a good man, an honest guy, well qualified in terms of his life experience, but I don't think he grew the vote today," Graham said. "I think there are a lot of Democrats that may have been thinking about voting for him are going to have a harder time of it."
Graham said Tillerson failed to answer a key question at this hearing about what he intends to do about Russian election-related hacking.
"Real fuzzy answer," Graham said. "He needs to clear that up. If he doesn't clear that up, it would be a problem, and I think he can clear it up and he needs to clear it up."
CNN Reports