As we enter the third day of hearings for Donald Trump’s nominees, Rex Tillerson appears to face the toughest confirmation fight of all of them. After a day of questioning Wednesday, Sen. Marco Rubio wouldn’t commit to voting for Tillerson, saying he was “prepared to do what’s right” even if fellow Republicans vote for the nominee for secretary of state.

Rubio had pushed Tillerson throughout the hearing to condemn human rights abuses in Russia, Saudi Arabia, and the Philippines, which the nominee stubbornly refused to do, insisting that he would wait until he had access to more information. He also pointedly asked Tillerson if Vladimir Putin is a war criminal. (Tillerson answered, “I would not use that term.”)

Tillerson’s long-standing ties to Russia as Exxon Mobil CEO were controversial even before this week’s round of Trump/Russia accusations. In his testimony, Tillerson did condemn Russia’s annexation of Crimea, called the country an “unfriendly adversary,” and said “we’re not likely ever to be friends” (a strange statement from a man who’s literally received Russia’s “order of friendship”). But it evidently wasn’t enough to convince Rubio.

Republicans only have a one-vote majority on the foreign-relations committee, so if Rubio and all 10 democrats vote against Tillerson, it could stall his nomination. The Senate can then bypass the panel and bring his nomination to the full chamber for a vote, where he would need only a simple majority. There, his nomination would be likely, but not guaranteed. South Carolina Sen. Lindsey Graham says he’s also still undecided. Blocking Tillerson would still require one more Republican (John McCain?) and all the Democrats to reject his nomination. As my colleague Josh Voorhees wrote earlier this week, that’s only happened nine times in the history of the Senate, most recently in 1989.