Rubio, a member of the Senate Foreign Relations Committee, drilled Tillerson on his longstanding business ties with Russia during Tillerson's marathon confirmation hearing Wednesday. Rubio, who was defeated by Trump in the Republican primary, repeatedly asked Tillerson whether he believes Russian President Vladimir Putin is a "war criminal." Tillerson refused to agree with Rubio's characterization. He did, however, acknowledge Russia as an "unfriendly adversary."
Two other leading Senate Republicans on issues of foreign affairs, John McCain and Lindsey Graham, have also expressed reservations about Tillerson's ties to Russia, a country the U.S. intelligence community has said directly interfered in the 2016 election to help Trump defeat Hillary Clinton.
None of the three lawmakers have said how they intend to vote on Tillerson's nomination.
Rubio, however, holds particular sway. Republicans control the Foreign Relations Committee by just one vote, meaning a single GOP defection could seriously hamper Tillerson's nomination.
Technically, the committee's vote is only an advisory one. Cabinet nominees can still be confirmed by the full Senate even if they do not receive the backing of the committee that held their confirmation hearing. However, those committee votes tend to hold significant sway over how the full Senate votes. The last Cabinet nominee to be confirmed after failing to win over the committee with jurisdiction was Henry Wallace, Franklin Roosevelt's former vice president who was bumped from the ticket in 1944 and was eventually nominated to become commerce secretary in Roosevelt's fourth term.