Sen. Marco Rubio said Wednesday he was not sure if he was going to vote to confirm Rex Tillerson as secretary of state because "I'm prepared to do what's right" — even if it means being the only Republican on the Senate Foreign Relations Committee to oppose President-elect Donald Trump's nominee.

"I intend to take this very seriously," the Florida Republican told reporters after Tillerson's daylong hearing. "I'm going to look at this, a very important decision."

"I recognize the partisan split of the committee and what it all means," he added. "I have to make sure that I'm 100 percent behind whatever decision that I make — because once I make it, it isn't going to change."

Rubio extensively grilled Tillerson during the hearing, attacking the retired Exxon Mobil CEO for refusing to call Russian President Vladimir Putin a war criminal for his army's role in the Syrian civil war and arguing over new bipartisan legislation calling for mandatory sanctions against Moscow for its meddling in the U.S. election and other aggressive behavior worldwide.

The senator also pressed Tillerson to acknowledge the U.S. intelligence community's assessment the Kremlin hacked into the Democratic National Committee and other party groups.

Rubio, who was re-elected to a second term in November, is one of 11 Republicans on the Senate's Foreign Affairs Committee. With 10 Democrats on the committee, Rubio would almost certainly be the swing vote if he votes against Tillerson. That would kill the nomination and prevent it from going to the full Senate for a vote.

After Trump announced Tillerson's nomination last month, Rubio was among several senators — Republican and Democrat alike — who raised concerns about his deep ties to Moscow and Putin.

"It's not an effort to embarrass anyone," Rubio told reporters of the questions he posed to Tillerson during the hearing. "This is a gentleman who didn't need to do this.

"He was headed for very comfortable retirement — and the only reason he's doing this is because he loves America and wants to serve it.

"I respect that deeply, and I wanted him to understand that.

"But I also wanted him to understand that these questions were designed for very specific reason — and that is that if we're going to have moral clarity in our foreign policy, we need to be clear.

"I don't want to see us move towards a foreign policy in which human rights only matters when nothing else matters, when something more important isn't standing in the way.

"I'm prepared to do what's right."

Rubio added his committee vote would be the same should Tillerson's nomination reach the full Senate.

"I'm not going to vote two different ways."