WASHINGTON -- All eyes are on Sen. Marco Rubio of Florida as a Senate committee is poised to vote on President Donald Trump’s nominee to be secretary of state.

The nomination of Rex Tillerson got a boost on Sunday after two influential Republican senators - John McCain of Arizona and Lindsey Graham of South Carolina - offered tepid endorsements of the former Exxon Mobil chief. The focus shifts to the Foreign Relations Committee on Monday afternoon as the members, including Rubio, cast their votes on Tillerson.

Rubio, whom Trump defeated for the GOP presidential nomination last year, clashed with Tillerson at a committee hearing earlier this month. Rubio bridled at his refusal to label Russian President Vladimir Putin a “war criminal” or condemn human rights violations in Saudi Arabia and the Philippines in strong enough terms. He chided Tillerson over the need for “moral clarity.”

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But as the committee prepares to vote on Tillerson’s nomination, Rubio faces a dilemma. He must either back down, which means a retreat from a strong stand on Russia and human rights, or cross Trump in the first days of his presidency.

A “no” vote from Rubio would not doom Tillerson’s confirmation, because the nomination could go directly to the Senate floor without a positive committee recommendation. But it would be an embarrassing rebuke to Trump just as his presidency gets underway, with questions swirling about his ties to Russia. GOP party activists would be certain to remember Rubio’s defection, although it’s impossible to predict how such a vote might look years from now if Rubio ever runs for president again.

Ahead of the vote Rubio was keeping fellow senators and everyone else guessing. After Trump was sworn in on Friday, Rubio was tight-lipped, saying only that he would make his decision “certainly before the vote” and that Tillerson’s responses to written questions had addressed some of his concerns.

Neither McCain nor Graham is on the committee, but their support could make it tougher for Rubio to remain a holdout. McCain said Tillerson’s responses to his questions, particularly in private, convinced him that the nominee could spearhead U.S. diplomacy.

“Though we still have concerns about his past dealings with the Russian government and President Vladimir Putin, we believe that Mr. Tillerson can be an effective advocate for U.S. interests,” McCain and Graham said in a joint statement on Sunday.

Their support makes Tillerson’s ultimate confirmation more likely in the narrowly divided Senate, where Republicans hold a 52-48 edge. The Foreign Relations Committee is narrowly split between 11 Republicans and 10 Democrats, so Republicans must all hold together to vote Tillerson out favorably unless he gets some Democratic backing.

Tillerson himself, former chief executive of Exxon Mobil, refused to respond to questions about Rubio’s opposition as he traversed the Capitol on Friday after attending Trump’s inauguration. Tillerson has a long record of doing deals in Russia and questioned U.S. sanctions against Russia, which has provoked concerns from Democrats and some Republicans over his selection.

Further roiling the debate is U.S. intelligence’s assessment that Russia meddled in the presidential election to help Trump defeat Hillary Clinton.