Rex Tillerson is on thin ice. And Marco Rubio put him there.

Fresh off being sworn in for a second term, the Florida senator began making his mark in the new Congress through a series of dramatic exchanges with the Secretary of State nominee.

Tillerson began his case for confirmation with a call for moral clarity in the world—but the case began to fall apart just as soon as Rubio started on the offensive. Rubio pressed Tillerson on whether Vladimir Putin was a war criminal due to Russia’s bombing campaign in Syria, and cited a number of Russian dissidents allegedly murdered by the autocrat.

Tillerson refused to label Putin a war criminal, and began a theme that would pop up continuously throughout the hearing.

“I do not have sufficient information,” Tillerson told Rubio, regarding those allegedly assassinated by the Russian regime.

Shot back Rubio: “None of this classified, Mr. Tillerson. These people are dead.”

Tillerson filibustered on other key issues, telling Rubio he didn’t have an independent opinion on putting Cuba back on the list of state sponsors of terrorism; the Philippines’ anti-drug campaign that has left thousands dead through extra-judicial killings; and whether Saudi Arabia was a human rights violator.

The former ExxonMobil CEO sensed that he was losing Rubio halfway through the hearing, telling the Florida senator, “There seems to be some misunderstanding that somehow I see the world through a different lens. And I do not. I share all of the same values that you share and want the same things for people the world over in terms of freedoms. but I’m also clear-eyed and realistic about dealing in cultures. These are centuries-long…cultural differences.”

This did not satisfy Rubio, who responded, in essence, that while the businessman had talked a big game on “moral clarity,” he had failed to substantiate it.

“That’s why I asked you about whether Vladimir Putin was a war criminal, something you declined to label him as,” Rubio said in the hearing. “I asked about China, whether they were one of the worst human rights violators in the world… I asked about the killings in the Philippines. I asked about Saudi Arabia being a human rights violator, which you also declined to label them. The reason was not because I was trying to get you involved in the international game of name calling. But in order to have moral clarity, we need clarity. We can’t achieve moral clarity with rhetorical ambiguity.”

Rubio has an enormous amount of leverage over Tillerson’s future: Given the makeup of the Republican-controlled Senate, the defection of just three GOP senators, combined with unanimous Democratic opposition, could doom Tillerson’s nomination. And this is a distinct possibility, considering the skepticism of the nomination from fellow GOP senators John McCain and Lindsey Graham.

And on the Senate Foreign Relations Committee, Republicans have a slim 11-10 member advantage—meaning that Rubio’s sole defection could stall the nomination in the committee process before it even gets considered by the whole Senate.

“I’m prepared to do what’s right,” Rubio told reporters after the hearing, after being asked if he was prepared to be a lone Republican holding up Tillerson’s nomination. “We’ll wait over the next couple of days and then we’ll make a decision…I think it’s important that if you’re going to stand for moral clarity, that you be clear.”

Tillerson didn’t perform well under intense grilling from other senators, either.

Tillerson stumbled repeatedly during an eight hour marathon of questions, appearing either unprepared or out of step with President-elect Trump on various issues. His missteps highlighted that he had spent his entire career in the private sector, and had never spent time working on foreign policy or national security issues.

On multiple occasions, Tillerson claimed not to have enough information to offer a response. More often than not, this answer made no sense.

Tillerson told senators that ExxonMobil had never lobbied against sanctions, despite clear lobbying disclosures that indicated the contrary. When questioned with the disclosures by Sen. Bob Menendez, Tillerson said he had not reviewed them—and given his background as CEO of the company, and the ongoing debate about sanctions in the Senate, it was strange that he had not been prepared for the question.

When confronted with publicly available information about human rights abuses abroad, Tillerson said he would not rely on press reports in the public domain.

“I’m going to act on factual information. I’m not going to act on what people write about in the newspapers,” he said, in one exchange with a senator.

He had tried to come off as a conventional Republican Russia hawk, telling the committee that “Russia today poses a danger,” but later said he opposed legislation that would automatically triggered sanctions in the wake of a cyberattack.

“What’s troubling about your answer is the implication that somehow if there is some country that we’re trying to improve relations with or have significant economic ties wih, you may advise the president not to impose sanctions…out of concerns that it could damage relations,” Rubio replied.

“We aren’t likely to be friends,” Tillerson said of Russia, despite the fact that he has actually received an “Order of Friendship” from the Russian government. Puzzlingly, he also said at one point he had talked to Trump about climate change, but had not discussed Russia policy.

The former ExxonMobil CEO also broke with Trump repeatedly on key issues: on the role of the NATO alliance, on nonproliferation of nuclear weapons, and on the proper way to view the Russian intervention in Ukraine, for example.

Ultimately Tillerson delivered an unimpressive performance, one that showed that he had not done his homework and was not able to charm Democrats predisposed to oppose him.

In one exchange, asked about ExxonMobil’s knowledge of climate change science, Tillerson suggested that the panel refer the question to the corporation he had headed since 2006.

“Do you lack the knowledge to answer my question or are you refusing to answer my question?” asked Democratic Sen. Tim Kaine, who had been Hillary Clinton’s running mate in the presidential race.

Tillerson answered, perhaps a little too cheekily: “A little of both.”