The President-elect is leaning towards entrusting the ExxonMobil tycoon with the stewardship of US diplomacy, a move that could ignite the first showdown between the pugilistic President-elect and senators in his own party.

The nomination -- which Trump tweeted will be announced Tuesday -- would be provocative, given Tillerson's personal relationship with President Vladimir Putin amid an uproar over CIA assessments that Russia likely intervened in the presidential election to help Trump.

I will be making my announcement on the next Secretary of State tomorrow morning.

The potential selection of a millionaire oil titan also offers insights into Trump's vision of diplomacy as akin to the wheeler-dealer world of big business -- and his disdain for concerns that the web of business connections surrounding him and future members of his Cabinet pose unacceptable conflicts of interest.

A Tillerson nomination would also exacerbate concerns about the level of formal foreign policy experience in Trump's outsider administration.

On the other hand, he's arguably better acquainted with world leaders than a potential pick for the secretary of state job who has spent decades toiling in traditional, lower-level diplomatic posts.

Any attempt to block Tillerson would ally Democrats critical of his Big Oil worldview and potential conflicts of interests with Republican Russia hawks with the stature to defy the nascent administration.

For John McCain, who branded the Russian leader a "thug and a murderer" in a CNN interview Saturday, Tillerson's cordial ties with Putin, whom the Arizona Senator considers a US enemy, are a red flag.

"It is a matter of concern to me that he has such a close personal relationship with Vladimir Putin," said the 2008 Republican presidential nominee on CBS' "Face the Nation" Sunday.

"And, obviously, they have done enormous deals together, that that would color his approach to Vladimir Putin and the Russian threat."

JUST WATCHED Trump: Rex Tillerson is a 'world-class player' Replay More Videos ... MUST WATCH Trump: Rex Tillerson is a 'world-class player' 03:08

Tillerson, 64, was a driving force behind ExxonMobil's partnership with Russian oil giant Rosneft and drilling projects in the Arctic, Black Sea and Siberia. In 2014, Putin awarded the ExxonMobil CEO the Order of Friendship, one of the highest honors Russia grants to foreign citizens.

McCain said that Tillerson would get a fair confirmation process if he were nominated. But he is not the only Republican senator to register alarm at the prospect of Tillerson at Foggy Bottom.

"Being a 'friend of Vladimir' is not an attribute I am hoping for from a #SecretaryOfState," Marco Rubio said in a tweet signed with his initials on Sunday.

Rubio railed against Putin's Russia during his own presidential campaign and an intervention in a Tillerson confirmation battle could help the Florida senator rehabilitate his reputation as a foreign policy heavyweight following his losing primary campaign.

Another key Republican in a Tillerson confirmation process could be South Carolina Sen. Lindsey Graham, who warned Sunday Russia was trying to "break the backs" of the world's democracies.

Given the 52-48 GOP advantage in the Senate, it would not take many Republican defections to doom Tillerson -- though the math would be complicated if Trump names a Democrat like Sen. Heidi Heitkamp of North Dakota or Sen. Joe Manchin of West Virginia to his Cabinet.

The swift umbrage expressed by some Republicans about Tillerson raises the possibility that the Trump team floated his possible nomination to gauge his chances of confirmation.

On the other hand, it is also possible that an assured performance in the hearing room could placate concerned Republicans.

But the ExxonMobil boss would complete a foreign policy triumvirate along with Trump and designated national security advisor retired Gen. Michael Flynn with no formal diplomatic pedigree. Flynn, however, is a veteran of military intelligence and played a key role in combating the Taliban insurgency in Afghanistan.

While such a set up is faithful to Trump's vows to blow up the Washington establishment, it could leave the administration exposed in the event of an international crisis, particularly early in the administration.

But Tillerson would be any easy fit with Trump's growing club of millionaires and billionaires around the Cabinet table, along with another detachment of retired military brass and a strong core of authentic movement conservatives.

The President-elect explained on Fox News Sunday why Tillerson has moved to the top of the list for secretary of state, ahead of such candidates as former GOP nominee Mitt Romney and Tennessee Senator Bob Corker.

"He's much more than a business executive. I mean, he's a world-class player. He's in charge of, I guess, the largest company in the world," Trump said. "To me, a great advantage is he knows many of the players, and he knows them well."

JUST WATCHED Sources: Tillerson leading choice for state Replay More Videos ... MUST WATCH Sources: Tillerson leading choice for state 00:55

Trump has a point that Tillerson, as the face of ExxonMobil's global business, has expertise in negotiating at the highest levels.

His contacts run deep in the Gulf and elsewhere in the Middle East, and ExxonMobil also has operations throughout Asia, which could witness a confrontation between the Trump administration and China.

ExxonMobil is sometimes compared to a de facto state in terms of its power and reach, and the head of such an influential energy firm would quickly founder without in depth knowledge of geopolitics and acquaintance with the domestic politics weighing on key world leaders.

And yet, Tillerson, if nominated, will have to satisfy senators that there is more to him than a businessman eyeing a new challenge in statesmanship.

Not much is known, for instance, about his personal views on international relations as distinct from the positions of his company.

Tillerson's attitude to the national security architecture the US has sponsored since the end of World War II will come in for scrutiny, as well as his views on alliances in Asia and Europe on which Trump has cast uncertainty.

He will have to explain why his own lack of formal foreign policy experience is not a disqualifying feature of his resume. After all, negotiating is just one aspect of the job of secretary of state. The top US diplomat must also frame a coherent foreign policy doctrine for the United States, exemplify its values, have a deep grounding in history and understand the complicated motivations of its international partners and adversaries.

It's one thing to negotiate with an autocratic leader over an oil contract. It's another to do so on a contentious foreign policy issue in a way that is consistent with bedrock US values, like support for human rights.

A confirmation hearing would also force Tillerson to develop positions on issues ranging from global warming to North Korea's nuclear program and US humanitarian aid in the developing world.

Environmentalists, already alarmed by Trump's nomination of Oklahoma Attorney General Scott Pruitt -- a fierce opponent of climate change regulation of power plants -- to head the Environmental Protection Agency, are mobilizing against a potential Tillerson nomination.

"By naming Rex Tillerson for Secretary of State, Trump would truly be turning over several key Cabinet positions to Big Oil," said League of Conservation Voters President Gene Karpinski in a statement.

"His appointment would cause a massive conflict of interest, giving ExxonMobil -- a company that investigative reports have shown covered up their own climate research and misled the public for decades -- an unprecedented ability to sway foreign policy to protect their own interests," he charged.

While ExxonMobil, like other energy firms, has incurred the wrath of environmental groups, it did back the Paris climate accord concluded by the Obama administration this year.

JUST WATCHED Cardin 'concerned' about Tillerson and Russia Replay More Videos ... MUST WATCH Cardin 'concerned' about Tillerson and Russia 01:46

Tillerson's personal views on the agreement are unclear, though he has said there is a human contribution towards climate change and backed a carbon tax. Trump has called climate change a "hoax" and suggested he could pull out of theinternational Paris agreement outlining individual emission limits for member countries.

If nominated, Tillerson would also have to confront claims that his position atop the State Department would represent an unresolvable conflict of interest given his past business dealings.

Even if he were to sever all links and investments related to ExxonMobil, action he took as secretary of state -- for instance, advocating the lifting of sanctions against Russia over its annexation of Ukraine -- could clearly benefit the firm.

New Jersey Democratic Sen. Bob Menendez laid out the terms of the brewing confirmation battle in a Facebook post on Saturday, calling reports of Tillerson's possible nomination "alarming and absurd."

"With Rex Tillerson as our Secretary of State, the Trump administration would be guaranteeing Russia has a willing accomplice in the President's Cabinet guiding our nation's foreign policy," Menendez wrote.

He continued, "The role of Secretary of State should not be bestowed upon someone whose only notable experience with foreign governments involve multimillion dollar deals with Russia and whose experience with the federal government seems to be limited to campaigns against the effects of climate change."

Tillerson might not be the only State Department confirmation battle percolating for Trump.

Kentucky Sen. Rand Paul said Sunday he would seek to block Trump's widely rumored pick for deputy secretary of state, former US ambassador to the United Nations John Bolton.

"I'll do anything to try to prevent John Bolton from getting any position because I think his worldview is naive," Paul said on ABC's "This Week."

"He's still a big cheerleader for the Iraq War," Paul added. "He's promoted a nuclear attack by Israel on Iran. He wants to do regime change in Iran."