Senator John McCain, a leading critic of Russia, says he'll support President-elect Donald Trump's pick for secretary of state when 'pigs fly.'

The Republican lawmaker's disapproval of Rex Tillerson, who once received a friendship award from Vladimir Putin, makes the former Exxon Mobil chairman's confirmation even murkier.

The GOP holds 52 seats in the U.S. Senate. They need a bare majority of 51 votes to approve the Trump's appointment.

Senator John McCain, a leading critic of Russia, says he'll support President-elect Donald Trump's pick for secretary of state when 'pigs fly'. Republicans disapproval of Rex Tillerson makes his confirmation murkier than it already was

Tillerson won over most of the Republican caucus during meetings at the U.S. Senate on Wednesday, but Democrats want more time to consider his record, especially his ties to Russia.

Senator Bob Corker, the Republican chairman of the Senate Foreign Relations Committee, described Tillerson as 'very much in the mainstream' of U.S. foreign policy thinking.

Tillerson, Exxon Mobil's former chief executive, drove the company's expansion in Russia and opposed sanctions imposed over its annexation of Crimea.

Many lawmakers, including Republicans, have expressed concerns about Tillerson's relationship with Moscow, given its differences with Washington not only over Ukraine but the civil war in Syria.

Marco Rubio has said he has 'serious concerns' about Tillerson. He'll cast the deciding vote on the Senate Foreign Relations Committee, where Republicans hold 10 seats to Democrats' 9.

Republicans can only afford a single defection in the full Senate, notwithstanding some Democratic support for Tillerson, or his his nomination won't be approved.

Asked on Wednesday if he could support Tillerson, McCain, a leading U.S. critic of Russia, told reporters: 'Sure. There's also a realistic scenario that pigs fly,' the Houston Chronicle reported.

Democrats want more time to consider his record, especially his ties to Russia. Senator Chris Coons, right, seen with Tillerson Wednesday, said next week may be too soon for the hearing

Republican Lindsey Graham has said he'll vote for Tillerson if he acknowledges that Russia interfered in the presidential election and supports sanctions against the country for its bad actions.

U.S. intelligence officials have concluded that Russia intervened in the 2016 election to help the Republican, Trump.

Trump says the intelligence is faulty and hacks on high-profile Democrats, including Hillary Clinton's campaign chairman John Podesta, could have been carried out by anyone.

'Julian Assange said "a 14 year old could have hacked Podesta" - why was DNC so careless? Also said Russians did not give him the info!' Trump said this week on Twitter.

The president-elect was supposed to get an intelligence briefing early this week on sanctions that were put in place by the Obama administration but it was delayed until Friday, timing Trump said was suspicious. He suggested his briefers needed 'more time' to build a case.

Corker, whose committee will conduct Tillerson's confirmation hearing, which is expected to start on Jan. 11, told reporters he was comfortable Tillerson would lead a robust U.S. policy toward Russia.

Senator Ben Cardin, the top Democrat on the foreign relations panel, said after his meeting with Tillerson he had not reached any conclusion on him.

'We're just at the beginning of the process,' Cardin told reporters after spending about an hour with the nominee.

Tillerson did not speak to reporters.

Senator Ben Cardin, the top Democrat on the foreign relations panel that will cast the first vote, said after his meeting with Tillerson he had not reached any conclusion on him

Republicans can only afford a single defection in the full Senate, notwithstanding some Democratic support for Tillerson, or his his nomination won't be approved. Lindsey Graham, left, has also said he may not vote for the Republican

Graham wants Tillerson to admit Russia carried out hacks on Democrats. Trump says the intelligence is faulty

Russia and Tillerson's view of sanctions are expected to be a focus of Tillerson's confirmation hearing, which could last for two days next week.

Democrats have called for a delay before Tillerson's hearing, given the complexity of his financial records and ties to Exxon after spending decades at the oil giant.

Cardin said it was too soon to discuss Tillerson's agreement, announced late on Tuesday, to sever all ties to Exxon Mobil to comply with conflict-of-interest requirements.

Senator Chris Coons, another Democrat on the foreign relations panel, said next week may be too soon for the hearing, given Republicans' plans to vote at the same time to repeal Democratic President Barack Obama's healthcare law.

'It strikes me as trying to get too many things done at the same time,' he said.

Coons said he was 'generally encouraged' by some of Tillerson's answers during their 1-1/2-hour meeting but had not decided whether to support his nomination.

The Senate Judiciary Committee has also scheduled a hearing for Jeff Sessions for attorney general next week and the Senate Intelligence Committee will hear from Trump's choice for CIA Director, Mike Pompeo.