Secretary of State-designee Rex Tillerson faced questions on Russia throughout a lengthy confirmation hearing Wednesday, but the most aggressive exchange came from a Republican senator who could sink his nomination in committee: Marco Rubio.

At the Senate Foreign Relations hearing, the fast-talking Floridian and former presidential candidate grilled the former Exxon Mobil CEO on sanctions against Russia and whether Russian president Vladimir Putin was a war criminal who murders his political opponents.

Rubio appeared frustrated when Tillerson said he needed more facts, including classified information, before he could speak about Putin and his associates being responsible for war crimes and killing opponents.

“None of this is classified, Mr. Tillerson,” Rubio quipped. “These people are dead.”

[Senators Warn Tillerson About Backing New Russia Sanctions]

Russia was expected to dominate the proceedings given Tillerson’s business ties to the country during his tenure at Exxon Mobil. Existing sanctions have restricted the oil conglomerate’s ability to develop and expand Russian oil fields, raising questions about his expected role in future Trump administration deliberations on whether and when to lift sanctions.

“While I do not suggest it was your intent, it is frankly not too great a distance from Exxon’s business partnerships to Putin’s Kremlin-controlled slush funds essential for his ‘disinformation’ campaigns around the world,” said Sen. Benjamin L. Cardin of Maryland, the committee’s top Democrat. 

But Rubio’s tough questioning was notable because the Foreign Relations Committee consists of 11 Republicans and 10 Democrats. Tillerson’s nomination could be blocked if Rubio breaks ranks and joins a unified Democratic roster in opposition.

[Can Republicans Afford to Oppose Tillerson?]

In a tense six-question exchange, Rubio also pressed Tillerson on whether he would advise President-elect Donald Trump to sign a bill imposing sanctions and visa restrictions on individuals who conducted cyberattacks against the United States.

Tillerson said the president should be given flexibility to factor in the country’s relationship with the U.S.

“Giving the executive the tool is one thing,” Tillerson said. “Requiring the executive to use it without any other considerations, I would have concerns.”

Rubio called Tillerson’s response “troubling” because a cyberattack, he said, is “a direct attack on our national security and our electoral process.”

The back and forth came one day after Rubio and nine other senators unveiled additional Russian sanctions legislation in response to what the intelligence community has concluded was Kremlin-directed hacking of the Democratic National Committee, and to the Russian aggression in Ukraine.

[Trump Facing Questions on Whether Russian Intel ‘Cultivated’ Him]

President Barack Obama imposed sanctions against several individuals last month, and Tillerson also declined to answer Rubio’s questions about whether he would advise Trump to repeal those sanctions issued by executive order.

Tillerson’s answers could also raise concerns among two other Republican senators and outspoken Kremlin foes: John McCain of Arizona and Lindsey Graham of South Carolina.

Graham told reporters Wednesday evening that he had not decided whether to support Tillerson’s nomination, but he said the nominee’s answer on sanctions and his hesitancy to call Putin a war criminal was “a little unnerving.”

“I’ve talked to [Tillerson], he gave pretty good answers so we’ll see,” Graham said. “I voted for all of Obama’s Cabinet officials. I want to support this president, but the Russia issue really matters to me. I don’t want to send a signal to Russia that somehow we’re going to be less than aggressive.”

Tillerson’s response on Russian sanctions also unsettled Democrats, who argued it could send a dangerous message to adversaries.

“To duck the question, and refuse to commit to continuing these sanctions is tantamount to sweeping Russia’s flouting of international norms under the rug,” Senate Minority Leader Charles E. Schumer said on the Senate floor at the same time as the hearing.

[Intelligence Officials Say Putin Ordered Campaign to Help Trump]

“It says, ‘Go ahead, interfere in our elections again. Nothing will happen to you,’” Schumer said. “It says the same to China, and Iran, and any other country that might try to hack.”

During a news conference Wednesday in New York, Trump declined to go into detail on steps he might take to respond to the Russian interference. He has previously raised doubts about the intelligence community’s consensus that Russia was behind the DNC hack, but admitted Wednesday that he believed the Kremlin was responsible.

As Trump spoke, Tillerson faced questions on Capitol Hill on how he might balance the incoming president’s social media missives on foreign policy. 

Indiana GOP Sen. Todd Young pointed out that Trump’s habit of tweeting could affect Tillerson’s credibility as the country’s chief diplomat.

“I don’t think I’m going to be telling the boss how he ought to communicate with the American people,” Tillerson said.

Asked if he had a “contingency plan,” the secretary of State nominee said, “I have his cell phone number and he’s promised me he’ll answer.”

To which Young responded, “We’ll hope for the best there.”