Sen. Marco Rubio sounded off about President-elect Donald Trump's likely secretary of state pick on Twitter Sunday morning.

'Being a "friend of Vladimir" is not an attribute I am hoping for from a #SecretaryOfState,' Rubio wrote, initialing the tweet to ensure Americans knew it was coming directly from the senator's mouth.

Rubio, a former political nemesis of the GOP president-elect who eventually endorsed him, was responding to the news that Trump planned to name ExxonMobil CEO Rex Tillerson as his top diplomat.

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Without naming names, Sen. Marco Rubio (left) expressed his worries about Donald Trump's (right) likely secretary of state pick Rex Tillerson

Sen. Marco Rubio used Twitter this morning to get the point across that he wouldn't vote for a secretary of state nominee with strong ties to Russian leader Vladimir Putin

ExxonMobil CEO Rex Tillerson will likely be named Donald Trump's secretary of state - even though he has no government or diplomatic experience

While Tillerson has no government or State Department experience, he does have a relationship with Russian leader Vladimir Putin because of his business dealings, as the energy company operates in more than 50 countries.

'Well, in his case, he's much more than a business executive, I mean he's a world-class player,' Trump said, promoting Tillerson on 'Fox News Sunday.'

'To me, a great advantage is he knows many of the players, and he knows them well,' Trump continued. 'He does massive deals in Russia. He does massive deals for the company – not for himself, for the company.'

But Tillerson's relationship with Russia is unnerving to some members of Congress, including Rubio and a handful of other Republicans, especially in light of the CIA's claims that Russian hackers interfered with the American election, to purposely benefit Trump.

Sen. John McCain, the GOP's 2008 presidential nominee, said on 'Face the Nation' that while he believed Tillerson should get a fair hearing, he wasn't enthusiastic about the oil boss' close ties to Russia.

Sen. Marco Rubio, a former political rival of Donald Trump's who eventually endorsed the GOP nominee, tweeted his reservations about Trump's likely secretary of state pick

'It's a matter of concern to me that he has such a close personal relationship with Vladmir Putin,' McCain said. 'And obviously they've done enormous deals together.'

'That, that would color his approach to Vladimir Putin and the Russian threat,' McCain pointed out.

He and Sen. Lindsey Graham partnered up with Democratic Sens. Chuck Schumer and Jack Reed and today called for an investigation into Russia's role in the election year hacks.

As for McCain's claims that Tillerson's business dealings with Putin could make him more pro-Russian than other potential secretary of state picks, Tillerson has gone on the record speaking out against Sanctions, as Russia's move into Crimea in 2014, and subsequent sanctioning from the Obama administration, derailed a $500 billion deal to drill in the offshore Arctic Kara Sea oil field, according to NBC News.

'We do not support sanctions, generally, because we don't find them to be effective unless they are very well implemented comprehensively and that's a very hard thing to do,' Tillerson said in May 2014.

'We always encourage the people who are making those decisions to consider the very broad collateral damage of who are they really harming with sanctions,' the oil boss added.

While Republicans maintained control of the U.S. Senate, Rubio and McCain's comments suggested that Tillerson's confirmation hearing might not be smooth sailing.

Tillerson would be fourth in line for the presidency if he's confirmed.

The Tillerson pick also comes after weeks of speculation that Trump might pick a former thorn in his side, 2012 GOP nominee Mitt Romney, for the job.

Trump and Romney met for the first time at the president-elect's Bedminster, New Jersey, golf club several days after the presidential election.

The two then had a high-profile dinner together in New York.

Rudy Giuliani, another name in the running to lead Foggy Bottom, pulled his name out of contention on Friday.