President-elect Donald Trump has picked Rex Tillerson as his nominee for Secretary of State. Here's what you need to know about Tillerson. (Thomas Johnson,Victoria Walker,Danielle Kunitz/The Washington Post)

THE NATURE of Donald Trump’s relationship with Russia is the most obscure and disturbing aspect of his coming presidency. Mr. Trump’s unwavering defense of Vladi­mir Putin’s regime against all criticism, and his breezy dismissals of a CIA finding that Moscow sought to tip the presidential election in his favor, raise critical questions. Does the president-elect merely happen to believe that he should begin by trying to cooperate with Mr. Putin, or is he driven by undisclosed personal or financial interests? Apart from the election controversy, does Mr. Trump accept the overwhelming evidence that Mr. Putin’s Kremlin is bent on disrupting and diminishing Western democracies, and the United States in particular, and will he resist that project?

The uncertainty Mr. Trump has raised around these questions, including among a number of congressional Republicans, inevitably shadows his possible nomination of ExxonMobil chief executive Rex Tillerson as secretary of state. Mr. Tillerson is evidently an able and seasoned executive who has managed one of the world’s largest companies and, in doing so, has gotten to know issues and leaders around the world. But one of those leaders is Mr. Putin, with whom Mr. Tillerson has cut enough deals over the course of two decades to be awarded Russia’s Order of Friendship. He has been a staunch opponent of sanctions against Russia, which his company once estimated had cost it $1 billion.

Mr. Tillerson might be considered a logical and qualified pick for a president seeking to rebuild constructive relations with Moscow. Or his nomination could augur a sellout by Mr. Trump of vital U.S. interests in Europe, the Middle East and elsewhere. Given the opacity of Mr. Trump’s ties to Russia, a consequence of his continuing refusal to release his tax returns and business records, senators are right to be wary of a Tillerson nomination.

Mr. Trump’s rejection of the finding that Russian agents meddled in the election looks particularly blinkered in light of reports that Mr. Putin’s regime has tried to intervene in numerous other Western elections — most recently in Germany. The president-elect may reasonably worry that Democrats will use evidence of interference to delegitimize his administration. But it is one thing for Mr. Trump to defend what remains a clear electoral victory; it is quite another to deny or ignore the Putin regime’s malevolent actions, which should be fully exposed and punished with bipartisan support.

Similarly, it is understandable that ExxonMobil would object to sanctions that blocked oil and gas megaprojects Mr. Tillerson negotiated with Mr. Putin. But those sanctions were adopted for a vital geopolitical cause: to punish Russia for its invasion of Crimea and eastern Ukraine, and to force a political settlement. In the face of Mr. Putin’s refusal to implement the peace plan he agreed to, the lifting of sanctions now would invite more Russian aggression, perhaps directed at NATO members.

If Mr. Trump nominates Mr. Tillerson, senators should seek clear statements recognizing the well-established facts about Russia’s belligerent behavior. For his part, the president-elect could ease many concerns by disclosing, once and for all, any investments or loans his companies have received from Russian firms and individuals. What should be unacceptable is for a new president to radically shift U.S. policy toward an established adversary while denying — or hiding — material facts.