The avalanche of disclosures that Donald Trump continued his business contacts with Russian operatives as he sought and won the American presidency in 2016 has brought the threat to his Oval Office tenure to a boiling point.

The guilty plea from Mr. Trump’s onetime fixer, Michael Cohen, to the charge of lying to Congress about Mr. Trump’s proposed Moscow Trump Tower real-estate deal illustrates the classic singing of a mob-culture “canary.”

Mr. Cohen’s crumbling after 70 hours of testimony before House and Senate Intelligence Committees fingers Mr. Trump, identified as “Individual 1” in his plea, for whom he lied “out of loyalty.”

Mr. Cohen’s statement if nothing else bolstered his earlier contention that he was willing “to take a bullet” for his friend, for whom, in gangland lingo, he long had served as consigliere on the shadier side of the Trump real-estate racket. Mr. Cohen may not be fitted with cement boots, as in gangster fiction, but he is likely to find himself in the slammer for a time for his loyalty.

As for Mr. Trump, Mr. Cohen’s serenade will fatten the incriminating evidence being meticulously assembled by Special Counsel Robert Mueller’s investigation into Russian elections meddling, as well as Mr. Trump’s various personal and campaign activities of questionable legality.

The new cache of Cohen testimony might well also ensnare the president’s son Donald Jr., his daughter Ivanka and her husband-in-law Jared Kushner, of whom the latter two remain as White House officials. If so, that fact could play heavily in how Mr. Trump seeks to extricate himself and family members from the sea of political and criminal woes that appear to be awaiting them.

Some House Democrats, about to be armed with subpoena power in various committees by the “blue wave” that will give them the majority in the House in January, may want to pursue impeachment against the president. Others, including prospective Speaker Nancy Pelosi, seem willing to await the actions of Mr. Mueller to build the case against Mr. Trump on criminal grounds. It’s a process likely to take many more months and invite challenges that a sitting president cannot constitutionally be subject to criminal charges.

In 1974, President Richard Nixon chose to resign rather than face certain impeachment, after a Senate committee documented his role in the cover-up of the Watergate break-in scandal, and Republican congressional colleagues informed him they lacked the votes to save his presidency.

Mr. Trump, by temperament and his history as a counter-puncher, could well fight efforts to remove him from office via impeachment or any other method. In any event, despite the recent damaging developments to Trump, the end still appears to be some way off, and the process is only beginning to cleanse the American political system of the corruption wrought by this manifestly unfit occupant of the Oval Office.

Mr. Trump offered a bizarre explanation for his romancing of Russian President Vladimir Putin, to whom he reportedly offered a penthouse apartment worth $50 million in a planned Moscow development, before the 2016 election. “There was a good chance that I wouldn’t have won,” he said, referring to the election. “In which case, I would have gone back into the business, and why should I lose lots of opportunities?”

So much for The Godfather’s single-minded commitment to serve the country in which he had been given such personal wealth and prominence. In a 2015 email found by the New York Times, a Trump associate, Felix Sater, wrote to Mr. Cohen: “Our boy can become president of the USA and we can engineer it. I will get all of Putin’s team to buy in on this, I will manage this process.”

Now Trump says the confessed liar Mr. Cohen is lying again in hope of getting a lighter prison sentence. Of the alleged Moscow hotel deal, he said: “This was a project that we didn’t do, that I didn’t do,” adding “there would be nothing wrong if I did do it.”

Fellow Americans, this was your president speaking.

Jules Witcover is a syndicated columnist and former longtime writer for The Baltimore Sun. His latest book is “The American Vice Presidency: From Irrelevance to Power” (Smithsonian Books). His email is juleswitcover@comcast.net.