Nominating Exxon Mobil CEO Rex Tillerson’s for secretary of State was Donald Trump’s boldest challenge to Congress. And it particularly stoked senior Republican senators’ worries about Tillerson’s close relations with Russian president Vladimir Putin.

One of those senators was Marco Rubio of Florida, one of Trump’s erstwhile rivals for the Republican nomination. Rubio is also one of 11 Republicans on the Senate’s foreign affairs committee. With 10 Democrats on the committee, all of whom are likely to oppose Tillerson, he potentially has the deciding vote on whether Tillerson even makes it to the Senate floor for a confirmation vote.

And unlike committee chair Bob Corker, who called Tillerson an “inspired choice”, Rubio made no bones about his concerns during Tillerson’s confirmation hearing today.

Asked whether Russian president Vladimir Putin could have authorized Russia’s election hacks, the oilman attempted to punt. That drew a withering reply from Rubio: “Mr Tillerson, you’ve engaged in significant business activities in Russia, so I’m sure you’re aware that very few things of a major proportion happen in that country without Vladimir Putin’s permission.”

Tillerson caved in, saying, “I think that’s a fair assumption.”

Next, when Tillerson was asked whether he’d support automatically punishing people or countries that carry out cyberattacks on the US, he tried to waffle. Rubio called it “troubling” that Tillerson might advise against sanctions on a country that had made a cyber attack on the US, just “out of concern that it could damage our relations with them.”

In another exchange, Tillerson insisted he’d need more information to say whether he’d want to repeal Obama’s executive order for new sanctions against Russia after the recent cyber-attacks. Rubio was incredulous. “[A]ll the executive order says is that certain individuals responsible for cyber actions against the United States will be sanctioned and you still need to examine whether that’s a good idea or not. Is that correct?”

There was a subtext to this: Earlier in the hearing, Tillerson had blamed Obama for being too “weak” in his response to Putin seizing Crimea from Ukraine in 2014. Obama’s response then had been to impose sanctions. Now, Rubio was implying, when a cyber-attack on the US itself was at issue, Tillerson was doubting whether even sanctions were necessary.

Rubio had further disdain for Tillerson’s refusal to say whether Putin was a “war criminal” or had committed war crimes. ”There is so much information about what’s happening in Aleppo,” Rubio said. “I find it discouraging your inability to cite that—which I think is globally accepted.”

But it was in his final comment that the Florida senator hit the height of his grandstanding. Tillerson had refused to answer a question on whether “Putin and his cronies” had ordered the killing of dissidents and political opponents. ”None of this is classified, Mr Tillerson,” said Rubio. “These people are dead. They’re political opponents.”

For once, Tillerson was unbowed. ”Your question was, ‘Are people directly responsible for that?’ I’m not disputing these people are dead.”

Reports suggest Rubio is leery of being the sole Republican senator to torpedo a Trump nominee. Today’s hearing may have just been his way of making a point before voting yes. But if this grilling was anything to go by, he’s not relishing the prospect.