FBI officials reportedly inquired into whether Mr. Trump, wittingly or unwittingly, had fallen under Russian influence in the 2016 presidential campaign and also whether his firing of Mr. Comey amounted to obstruction of justice.
The New York Times and The Washington Post reports of the investigation drew a heated denial from Mr. Trump. He called a reporter's question about the allegation "the most insulting thing I've ever been asked." Secretary of State Mike Pompeo called the charge "absolutely ludicrous."
But the startling accounts have fanned impeachment talk that leading congressional Democrats up to now have preferred to defer. They would rather focus on the president's sea of domestic woes, generated by his refusal to end the longest government shutdown ever in hopes of compelling Congress to acquiesce to the southern border wall he demands.
Other prominent Democrats in Congress now holding committee subpoena power, however, are finding the temptation to use it too compelling not to target Mr. Trump for removal from office. On ABC's "This Week" Sunday, Rep. Adam Smith, a Washington Democrat argued: "Impeachment is an unbelievably serious undertaking. ... If the crimes are serious enough, it needs to be done."
The reported FBI counterintelligence on the sitting president was prompted by his own words and actions relating to Russian President Vladimr Putin. The Post recounted, for example, that Mr. Trump seized the notes of his interpreter after his Helsinki tete-a-tete with Mr. Putin, declined to make them public and ordered the interpreter's silence on what was said.
The new Democratic chairman of the House Intelligence Committee, Rep. Adam Schiff of California, has tweeted about his Republican colleagues now in the minority: "Will they join us now? Shouldn't we find out whether our president is really putting 'America First?' "
Mr. Trump, meanwhile, insists that nothing untoward was said at that Helsinki meeting with Putin. "We were talking about Israel and lots of other things, and it was a great conversation," he said. "I'm not keeping anything under wraps. I couldn't care less."
His steadfast party ally and golf buddy, Sen. Lindsey Graham of South Carolina, the new chairman of the Senate Judiciary Committee, came down hard on the FBI for its inquiry into the Trump-Putin relationship. He told "Fox News Sunday": "It tells me a lot about the people running the FBI. ... I don't trust them as far as I can throw them."
Much of the controversy goes back to another one-on-one conversation, this one between the newly elected Mr. Trump and FBI director James Comey. The latter reported that the president had cleared the room before asking him to let National Security Adviser Michael Flynn off the hook for lying about meeting with Russian Ambassador Sergey Kislyak.
If true, that request appeared to be a clear attempt at obstruction of justice, an impeachable offense. Mr. Trump subsequently fired Mr. Comey, telling ABC News host Lester Holt that he did so only because he believed he was not the right man to run the FBI. Mr. Comey then went public in his opposition to Mr. Trump, amid the trappings of a political vendetta between the two men that seemed to roil much of the nation's prime law-enforcement agency.
Now the two major political parties are split over the survival of the investigation of Justice Department Special Counsel Robert Mueller, himself a former FBI director.
Mr. Trump's new nominee for attorney general, William Barr, has said he has confidence in Mr. Mueller and will allow his investigation to go forward, as the Democrats are demanding.
One of them, Sen. Mark Warner of Virginia, the senior minority member of the Senate Intelligence Committee, says "the defining question" of the Mueller inquiry now is "Was there collusion?" between Mr. Trump and his 2016 campaign and the Russians, amid the new FBI disclosures.
Thus does the circle appear to be closing on the long quest for proof not only of the president's culpability in the despoiling of an American presidential election, but also in acts warranting his impeachment and conviction under Article II, Section 4 for "Treason, Bribery or other high Crimes and Misdemeanors."
Jules Witcover is a syndicated columnist and former long-time writer for The Baltimore Sun. His latest book is "The American Vice Presidency: From Irrelevance to Power" (Smithsonian Books). His email is firstname.lastname@example.org.