Former ExxonMobil chief executive Rex Tillerson's bid to be the next secretary of state received a major boost when a pair of Republican senators who had expressed consternation about him announced they will vote to confirm him, a move that effectively clears the way for him to win confirmation.

Senators John McCain and Lindsey Graham, two traditional GOP hawks who have voiced scepticism about Tillerson's ties to Russia, released a joint statement saying that after much thought, they have decided to back him.

"After careful consideration, and much discussion with Mr Tillerson, we have decided to support his nomination to be Secretary of State," they said. "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 US interests."

McCain first announced his decision on ABC. He said that it "wasn't an easy call".

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Republicans hold a 52-48 advantage over Democrats in the Senate, leaving Tillerson little margin for losing Republican support if every member of the Democratic Caucus aligns against him. McCain and Graham were seen as two of a small handful of Republicans who might defect.

But Tillerson backers can now breath a sigh of relief. Barring any surprise Republican defections, he will have the simple majority support he needs.

Before a full floor vote occurs, the Senate Foreign Relations Committee will vote on Tillerson tomorrow. The biggest question mark in the committee vote is Senator Marco Rubio, who grilled Tillerson about Russia during his hearing and seems dissatisfied with some of his responses.

Rubio has not said what he will do in the committee vote or in a Senate floor vote. But if he and all the Democrats on the committee vote against Tillerson, it would tilt the committee against him, dealing a humiliating blow.

Even in that scenario, however, there are ways to bring Tillerson's nomination to a full floor vote. Senate Foreign Relations Committee Chairman Bob Corker said last week that one way or another, Tillerson will "definitely come out of committee". Corker has also expressed confidence that Tillerson will be confirmed by the full Senate.

"When there's doubt, the president, the incoming president, gets the benefit of the doubt," McCain said on ABC, explaining his decision. "And that's the way I have treated every president that I have had the obligation to vote for or against as a member of the United States Senate."