President-elect Donald Trump has picked as his secretary of state Rex Tillerson, the chief executive of ExxonMobil, setting up a possible confrontation with members of his own party in the Senate, according to a person involved in the transition.

Since Tillerson’s name emerged as a candidate for the post, leading Republicans have expressed reservations about his years of work in Russia and the Middle East on behalf of the multinational petroleum company.

GOP advisers have warned that a growing number of Republican senators may be unwilling to vote to confirm Tillerson because of his ties to Russia. While Senate Democrats cannot filibuster Trump’s Cabinet picks, Republicans have only 52 votes in the Senate, leaving them in potential jeopardy if Democrats unite in opposition to Tillerson. It will take at least 50 votes to confirm a nominee, plus Vice President-elect Mile Pence casting a tie-breaking vote.

Yet Trump, after a protracted selection process that saw him also considering 2012 presidential candidate Mitt Romney, has decided to press ahead with Tillerson. Like others in the new Trump Cabinet, the ExxonMobil chief executive lacks any experience in government but will try to apply his experience in the business world to the realm of diplomacy. And he has worked extensively around the globe and built relationships with such leaders as Russian President Vladimir Putin.

The Trump team is planning an aggressive public relations campaign to win confirmation for Tillerson and dispel what it sees as a false narrative about his ties to Russia, a person involved in the transition said. Former secretaries of state Condoleezza Rice and James Baker are planning to go public Tuesday morning with their support for Tillerson, as is former defense secretary Robert Gates. Former vice president Richard B. Cheney also is supportive and may advocate for his confirmation.

Gates was the first person to raise Tillerson as a secretary of state possibility with Trump during a meeting at Trump Tower, the transition official added. Trump did not know much about Tillerson but started chewing over the idea. He invited Tillerson for a meeting and the two global deal makers hit it off. They recognized similarities in each other, and the more they talked, the more they liked each other, the transition official said.

Rice, who has served on the board of Chevron, then became a strong advocate for Tillerson. She and Trump spoke about Tillerson by phone Monday as Trump made his final decision.

One argument that the defenders of Tillerson – who during his career has also cultivated leaders of countries such as Saudi Arabia, Yemen, and Qatar – will probably make is that he stands firm in business negotiations in Russia and elsewhere.

“One of the things I know about the Russian government: I’m very predictable. And they know if I say no it means no. And talking about it more isn’t going to change that. No is still going to be no,” Tillerson said in a talk last year at the Texas Tech business school. “Over the years we’ve earned each other’s respect. Then when you say yes, you know we’ll follow through. It means something.”

Weighing whether to lift economic sanctions on Russia will be one of the first things on Tillerson’s plate, given Trump’s desire to smooth relations with the Kremlin. International economic sanctions, imposed after Russia annexed Crimea and gave support to insurgents in Ukraine’s eastern provinces, have fallen heavily on financial institutions and ExxonMobil.

ExxonMobil, which has a profitable operation on Sakhalin island in eastern Russia, had begun a drilling program in the Arctic’s Kara Sea, where Exxon made a find, and had agreed to explore shale oil areas of West Siberia and deep waters of the Black Sea. If sanctions are lifted, Tillerson told analysts this year, the Black Sea drilling would probably be the first to be restarted.

While ExxonMobil complained privately to the Obama administration about the sanctions, the company has abided by the law.

“They understand the situation. We understand the situation,” Tillerson said of the Kremlin when asked at an oil analysts’ meeting this year about whether Exxon would resume work in Russia if sanctions were lifted.

In addition, Tillerson will have to deal with climate issues because the State Department is the lead agency in international climate negotiations. Unlike Trump, Tillerson has said that he believes that climate change is real and has favored a revenue-neutral carbon tax of more than $20 a ton.

But environmental groups charge that Exxon knew about the harmful effects of fossil fuels as much as 40 years ago and failed to inform investors and the public, possibly in violation of securities laws. The New York and Massachusetts attorneys general and a range of non-governmental organizations are locked in battle over the charges.

Human rights experts are also unhappy about Tillerson’s nomination, noting that ExxonMobil does business in countries ruled by autocrats or dictators including countries in the Middle East, Equatorial Guinea and Kazakhstan.

“I don’t think that companies’ role is to play politics,” said Pavel Molchanov, oil analyst at the investment firm Raymond James. “They’re there to invest in resources. Saying that he personally has some special feelings toward Russia just because Exxon has invested there is probably overstating the case.”

But that might not be the way lawmakers see it.

At least four Republican senators have already publicly expressed their concerns with Tillerson’s Russia ties. Sen. Lindsey Graham, S.C., called the fact that Putin awarded Tillerson the Kremlin’s Order of Friendship in 2012 “unnerving,” while Sen. John McCain, Ariz., questioned Tillerson’s judgment on CNN on Monday noting, “I don’t see how anybody could be a friend of this old time KGB agent,” referring to Putin.

Sen. Marco Rubio, Fla., also tweeted over the weekend that “being a ‘friend of Vladimir’ is not an attribute I am hoping for from a Secretary of State,” while a spokesman for Sen. James Lankford, Okla., said he “has a lot of questions about Mr. Tillerson and his ties to Russia.”

Of the four, only Rubio sits on the Senate Foreign Relations Committee, which first must approve Tillerson’s nomination before it can head to the floor. Committee chairman Bob Corker, R-Tenn., who was in the running to be Trump’s Secretary of State, tweeted over the weekend that Tillerson “is a very impressive individual.”

But Republicans outnumber Democrats by only one on the Senate Foreign Relations Committee, giving Democrats an opportunity to block one of Trump’s most important Cabinet picks if they stay united in voting Tillerson’s nomination. Democrats would not say if they expect their committee members to hold rank. But if they can, they only need one Republican to vote against Tillerson’s nomination to keep him from proceeding to the Senate floor for a full confirmation vote.

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The Washington Post’s Paul Kane contributed to this report.