President-elect Donald Trump's nominee to head the State Department ran into some tough sledding from an unexpected source at Wednesday's confirmation hearing, as Senator Marco Rubio (R-FL) roasted Rex Tillerson over his refusal to call Russian President Vladimir Putin a "war criminal."

As we have reported, Secretary of State-designate Rex Tillerson faces many challenges to his confirmation, and one of the biggest was on full display at Wednesday’s hearing. Despite endorsing now-President-elect Donald Trump after calling him “dangerous” during the presidential campaign, Senator Marco Rubio came out swinging against Tillerson over his ties to Russia.

Rubio’s exchanges with Tillerson were tough. He directly asked Tillerson if Putin was a “war criminal,” to which Tillerson replied, “I would not use that term.” In response, Rubio gave a powerful account of the tragic situation in Aleppo, Syria, where Putin “has directed his military to conduct a devastating campaign… He’s targeted schools and markets and other civilian infrastructure. It’s resulted in the death of thousands of civilians.” Rubio also noted Putin’s bombings of Chechnya in the late 1990s, which killed an estimated 300,000 civilians, and also increased Putin’s approval ratings.

This was followed by an extended back-and-forth over whether Tillerson would describe Russian President Vladimir Putin as a “war criminal,” in which Tillerson seemed largely unmoved:

RUBIO: So, based on all this information, and what’s publicly in the record about what’s happened [inaudible], you are still not prepared to say that Vladimir Putin and his military have violated the rules of war, and have conducted war crimes in Aleppo?

TILLERSON: Those are very, very serious charges to make, and I would want to have much more information before reaching a conclusion. I understand there is a body of record in the public domain. I’m sure there’s a body of record in the classified domain, and I think in order to deal with a serious question like this —

RUBIO: Mr. Tillerson, what’s happened in Aleppo is in the public domain, the videos and the pictures are —

TILLERSON: — I would want to be fully informed before advising the president.

[…]

RUBIO: Mr. Tillerson, do you believe that Vladimir Putin and his cronies are responsible for ordering the murder of countless dissidents, journalists, and political opponents?

TILLERSON: I do not have sufficient information to make that claim.

RUBIO: Are you aware that people who oppose Vladimir Putin wind up dead all over the world, poisoned, shot in the back of the head. Do you think that was coincidental, or do you think that it is quite possible, or likely as I believe, that they were part of an effort to murder his political opponents?

TILLERSON: People who speak up for freedom in regimes that are repressive are often at threat. And these things happen to them. In terms of assigning specific responsibilities, I would have to have more information. As I indicated, I feel it’s important that in advising the president, if confirmed, that I deal with facts, that I deal with sufficient information, which means having access to all information. And I’m sure there’s a large body of information that I’ve never seen that’s in the classified realm. I look forward, if confirmed, to becoming fully informed. But I am not willing to make conclusions on what is only publicly available or has been publicly reported —

RUBIO: None of this is classified, Mr. Tillerson. These people are dead. Political opponents —

TILLERSON: Your question was people who were directly responsible for that. I’m not disputing these people are dead.

Although there are many glaring problems with Tillerson’s nomination, Rubio’s grilling may weigh heavier than most, because it hits him in an area of particular weakness with some Republican senators, whose votes he needs to be confirmed. If Republicans begin to peel off, his nomination will be in jeopardy.

Democrats have already left their mark on Tillerson’s nomination, but it is his consistent refusal to denounce Vladimir Putin that could finish him off, as Rubio has exposed Tillerson’s fundamental inability to stand up to Putin, or to President-elect Trump.