WASHINGTON (AP) — President Donald Trump’s nominee for secretary of state headed for approval in a key Senate committee Monday after Florida Sen. Marco Rubio announced his support, backing off from a challenge to the new president.

Rubio said that despite serious reservations about former Exxon Mobil CEO Rex Tillerson, particularly over his views on Russia, he believed a president was entitled to significant deference in assembling his Cabinet.

“Despite my reservations, I will support Mr. Tillerson’s nomination in committee and in the full Senate,” said Rubio, who’d come under strong pressure from fellow Republicans to back the nomination and avoid dealing Trump an embarrassing setback in the early days of his presidency.

Rubio’s announcement in a statement posted on Facebook came just hours before the Foreign Relations Committee was slated to meet and vote on Tillerson’s nomination. Rubio’s support virtually assures that the nominee will move through the committee and win full Senate confirmation.

Rubio had clashed with Tillerson at a committee hearing earlier this month, bridling at his refusal to label Russian President Vladimir Putin a “war criminal” and his failure to condemn human rights violations in Saudi Arabia and the Philippines in strong enough terms. He chided Tillerson over the need for “moral clarity.” But in the end, after unsuccessfully opposing Trump for the GOP nomination last year before coming around to support him, Rubio decided to fall in line this time, too.

His statement came after the nomination got a boost on Sunday from two influential Republican senators, John McCain of Arizona and Lindsey Graham of South Carolina, who also offered tepid endorsements for Tillerson. Like Rubio, McCain and Graham had voiced concerns in light of Tillerson’s long history of personal dealings with Putin, his record of doing oil deals in Russia and his questioning of the U.S. sanctions on that country.

It looks unlikely, though, that Tillerson will get much if any Democratic support in Monday’s vote, which would be unusual for a secretary of state nominee. Recent nominees for the job have been approved by overwhelming votes from both sides in the Foreign Relations Committee as senators have traditionally wanted to give a bipartisan vote of confidence to the nation’s top diplomat.

Sen. Ben Cardin of Maryland, the committee’s top Democrat, said in a statement Monday he won’t vote for Tillerson. Cardin said he based his opposition on Tillerson’s unwillingness to call Russia and Syria’s atrocities war crimes, or to describe Philippine President Rodrigo Duterte’s extrajudicial killings as gross human rights violations.

Cardin also said Tillerson misled the committee about Exxon’s lobbying against the sanctions imposed on Russia for its annexation of Crimea and support for separatists in eastern Ukraine.

“I believe Mr. Tillerson’s demonstrated business orientation and his responses to questions during the confirmation hearing could compromise his ability as secretary of state to forcefully promote the values and ideals that have defined our country,” Cardin said. Most of the other Democrats on the committee have also announced their opposition to Tillerson.

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

Even while announcing his support for Tillerson, Rubio laid out a series of concerns in his statement Monday.

“Despite his extensive experience in Russia and his personal relationship with many of its leaders, he claimed he did not have sufficient information to determine whether Putin and his cronies were responsible for ordering the murder of countless dissidents, journalists and political opponents,” Rubio said of Tillerson. “He indicated he would support sanctions on Putin for meddling in our elections only if they met the impossible condition that they not affect U.S. businesses operating in Russia.”