WASHINGTON - Congress is preparing to do major battle next month over President-elect Donald Trump's secretary of state nominee Rex Tillerson in a brewing proxy fight over Trump's potential financial conflicts of interests and his unconventional foreign policy.

The official sparring is expected to take place on Jan. 11, when the Senate Foreign Relations Committee plans to hold a two-part hearing to grill Tillerson over his tenure as Exxon Mobil's CEO and what many lawmakers believe are unseemly links to Russia.

But behind the scenes on Capitol Hill, the strategizing has already begun.

Democrats do not believe they stand much chance of stopping Tillerson from being confirmed. But they plan to press the top diplomat-designate to commit to full divestiture from Exxon and detail how he would pursue a different approach to the world as secretary of state, in what one Senate Democratic aide who requested anonymity to candidly discuss strategy called a "very thorough, tough vetting." As they have with Trump, Democrats plan to focus sharply on potential conflicts of interests deriving from Tillerson's finances, as well as his unorthodox views on critical foreign policy matters.

It's a grilling they would happily serve Trump, if only they had reason to drag him before a congressional committee.

Already, the minority party is railing against Republicans for refusing to demand Tillerson produce his tax returns. Their frustration stems from Tillerson's response to a routine question about tax returns on the Foreign Relations Committee's pre-confirmation hearing questionnaire. According to a Senate Democratic aide and a Trump transition team spokesman, Tillerson said he stood ready to provide only "tax return information" for his wife and himself for the previous three years. That means it's possible Tillerson isn't willing to provide complete returns.

Democrats have demanded that all Trump's Cabinet nominees release their tax returns — something Trump himself steadfastly refused to do during the campaign because, he said, the returns were under routine audit.

Foreign Relations Chair Bob Corker (R., Tenn.) said the GOP would not break with years of precedent to compel Tillerson to turn over his tax returns without good reason, no matter how loud the clamor from Democrats. "Unless there was a case where there's some irregularity that's known, the committee just does not ask for tax returns," Corker said in an interview.

Corker stressed that Tillerson is "not going to be the proxy [for Trump] because we're not going to ask for tax returns."

"I understand there are a number of Democrats who are very upset about President-elect Trump not putting his tax returns," Corker said. "But that's not going to change the way we go about our committee process."

Senate Democrats believe they can fight miniature versions of the political battles they are waging with Trump during the confirmation hearings for his Cabinet choices — particularly those like Tillerson who hail from the private sector. They intend to focus on Tillerson's four-decade career at a major oil conglomerate and what they — and some Republicans — see as questionable ties to Russian President Vladimir Putin. Several Democratic members of the committee have said they also plan to ask Tillerson about human rights and climate change in light of his Exxon work.

Many lawmakers are leaning on Trump to fully divest from his companies because his wide-ranging business empire lends itself at the very least to an appearance of conflict of interest. Democrats argue Tillerson's business career raises similar questions.

Just how thorny those conflicts may be won't be clear until the committee receives Tillerson's financial disclosure statements, documents that Corker expects to have before the end of 2016. Corker plans to give other committee members a copy of the disclosures "as soon as we have it," providing ample time to review what is expected to be a "very extensive" document.

If confirmed, Tillerson would be the first secretary of state in decades that has not previously worked as a public servant or member of the military.

"This proposed nominee for secretary of state will likely be asked more questions than normal. Secretary Kerry was somebody that we knew, Secretary Clinton was somebody that we knew," Corker said. "Since [Tillerson] will be up under the hood advising the president on foreign policy, a president who hasn't been involved in foreign policy in his business life, we certainly want to understand fully the kind of advice he would be giving the president as secretary of state."

Part of the logic behind planning a two-part hearing (one session in the morning and one in the afternoon), as Corker put it, is so there can be "plenty, plenty, plenty of time for people to learn everything" they want to from Tillerson.

A particular point of concern for Democrats and Republicans will be Tillerson's relationship and business dealings with Putin, who awarded him the Kremlin's Order of Friendship in 2013. Tillerson also opposed sanctions that the United States imposed on Russia in 2014 as a reaction to the annexation of Crimea and Moscow's support of separatists in eastern Ukraine, arguing that Exxon lost $1 billion from the policy change.

Last Thursday, Corker had a 30-minute conversation with Tillerson in which he questioned him about Russia, with the bulk of that conversation focusing on sanctions. He came away with the impression that Tillerson had no interest in ripping up existing sanctions on Russia, even if he criticized them as Exxon's CEO.

"That would be hugely problematic to this one senator, if I felt that was how we were going to begin the relations with Russia," Corker said, referring to himself. He noted the conversation "alleviated some of the concerns" he harbored about Tillerson's relationship with the Kremlin.

Corker said he encouraged Tillerson to start calling other lawmakers on his committee. Tillerson and lead Foreign Relations Democrat Ben Cardin (Maryland) also spoke by phone last Thursday, but a spokesman for Cardin would not detail what was discussed.

Tillerson was in Washington, D.C., on Tuesday, where he met with Sen. Tom Cotton (R., Ark.), who endorsed his nomination.

"I think, at least I hope, that when these meetings take place that we're going to see a Rex Tillerson that has a standard view of Russia's malicious activities, nefarious activities," Corker said. "I got a good sense of how he'd [approach] Russia and Putin and it was certainly different, if you will, than the picture of him toasting Putin with champagne."

Several key GOP senators — including John McCain (Arizona), Lindsey Graham (South Carolina), and Marco Rubio (Florida) — have expressed concerns that Tillerson's ties to Russia could be a dangerous echo of Trump's seeming affinity for Putin. Those concerns have taken on even more urgency in wake of CIA allegations that Russia attempted to tilt the election to Trump.

Since the president-elect selected Tillerson, Trump's transition team has been rolling out foreign policy heavyweights like Bob Gates, Condoleeza Rice and Dick Cheney to vouch for Tillerson's global bona fides. So far, only one Republican on the Foreign Relations committee — Rubio — has publicly criticized Tillerson over his Kremlin ties, and communicated those concerns directly to Corker before the nomination was formally announced.

Democrats need only convince one Republican to oppose Tillerson in order to block his nomination in committee. But it is still possible he could get to a floor vote, even if the committee rejects him. In the past, the Senate Foreign Relations Committee has reported out nominations unfavorably, a procedure that expresses the members' discontent without blocking the full Senate from voting. The full Senate confirmed in 1983 Reagan Arms Control and Disarmament Agency nominee Kenneth Adelman, despite his rejection by the Foreign Relations panel.

But Corker expressed confidence that Tillerson would pass the committee's test, even if his track record as an oil company CEO gives members pause.

"My sense is, based on the conversation we had, he's going to very much understand that his job now is to pursue our country's national interest," Corker said.