President Trump arrives at the Capitol to pay his respects to former president George H.W. Bush on Monday night. Trump spent Monday morning tweeting about the investigation into his 2016 campaign. (Alex Edelman/AFP/Getty Images)

For well over a year now, evidence has steadily accumulated that President Trump may have obstructed justice in connection with the investigation into his 2016 campaign. But Monday, Trump edged closer to an open display of obstruction and witness tampering: He urged potential witnesses against him to refuse to cooperate with law enforcement — and implied threats against those who do.

Trump began by publicly attacking Michael Cohen, his former attorney and fixer, who pleaded guilty last week to lying to Congress about a Trump real estate project in Moscow and who has been cooperating with special counsel Robert S. Mueller III.

Trump suggested in a series of tweets not only that Cohen is lying but also that he should receive no benefit for cooperating, as Cohen’s lawyers have requested: “‘Michael Cohen asks judge for no Prison Time.’ You mean he can do all of the TERRIBLE, unrelated to Trump, things having to do with fraud, big loans, Taxis, etc., and not serve a long prison term? … He lied for this outcome and should, in my opinion, serve a full and complete sentence.”

Obstruction and witness-tampering law prohibits retaliation by the subject of a criminal probe against those testifying against him. Trump is, of course, an identified subject of the relevant investigations. But as president, he is also ultimately responsible for the federal prosecution power — and he is openly attacking Cohen, who soon faces sentencing, with federal prosecutors who ultimately report to Trump given an important say.

Lest there be any doubt about Trump’s intent, his tweets against Cohen were shortly followed by comments on another potential witness: Roger Stone, a longtime confidant of the president. After excoriating Cohen, Trump quoted Stone in a tweet: “‘I will never testify against Trump.’” The president continued: “This statement was recently made by Roger Stone, essentially stating that he will not be forced by a rogue and out of control prosecutor to make up lies and stories about ‘President Trump.’ Nice to know that some people still have ‘guts!’ ”

So where he threatened a stick against Cohen, Trump offered a carrot to Stone, signaling where their allegiances should lie. This proof of potential witness tampering and obstruction of justice is made even stronger by the fact that the messaging is from the person who holds the most powerful get-out-of-jail-free card: a presidential pardon.

Taken together, Trump’s statements Monday are a message not only to Cohen and Stone, but also to anyone else who may be considering testifying or cooperating against the president or any of his associates. And the message is quite clear: Those who cooperate with law enforcement and agree to be witnesses against Trump will be punished, while those who keep his secrets will be protected.

The tweets Monday are not, alas, isolated incidents. The president has tweeted before that he is being persecuted by the special counsel’s office, purportedly threatened those in a position to preserve the Mueller investigation, asked for loyalty of those who might prosecute his confederates and engaged in a host of additional conduct evincing an intent to block Mueller’s work, as well as committing overt acts seemingly directed at obstructing the investigation.

For example, there are the recent Trump tweets in response to news that his former campaign chairman Paul Manafort’s cooperation deal with the special counsel’s office had collapsed: “Wait until it comes out how horribly & viciously they are treating people, ruining lives for them refusing to lie.” He added that Mueller “is doing TREMENDOUS damage to our Criminal Justice System, where he is only looking at one side and not the other. Heroes will come of this, and it won’t be Mueller and his . . . terrible Gang of Angry Democrats." Here too, Trump put a fine point on his meaning, soon adding that a pardon for Manafort was “not off the table.”

Trump may even have more directly encouraged witnesses to embrace presidential omertà — his version of the Mafia’s infamous code requiring its members to stay silent about its crimes. There have been reports, for example, that Trump may have had such conversations with Cohen before he initially pleaded guilty. If so, Mueller now knows the details, and we can expect them to emerge in due course.

Under normal circumstances, someone engaging in this type of behavior could be criminally charged. But in this case, the Justice Department’s Office of Legal Counsel has previously indicated that a sitting president should not be indicted. We disagree, and the issue is unresolved: Neither the Supreme Court nor any lower court has yet addressed it. Still, Mueller is unlikely to defy the department’s guidance.

That makes it all the more important that Congress examine whether the pattern of evidence constitutes an abuse of power or criminal obstruction of justice. That has proved a forlorn hope with Congress in the hands of Trump’s party. But with the House about to change hands, there is reason to hope that will occur once Mueller issues his long-awaited report on obstruction.

The president’s frenzied tweets suggest he recognizes that threat. Perhaps that’s why a later tweet in Monday’s tirade was directed at the special counsel: “Bob Mueller (who is a much different man than people think) and his out of control band of Angry Democrats, don’t want the truth, they only want lies. The truth is very bad for their mission!” Quite the opposite is the case: Congress, and all of America, await Mueller’s report on Trump’s deeply troubling behavior.

Read more:

Trump says collusion isn’t a crime. He’s right. It’s actually many crimes.

Trump’s real problem is that he obstructed justice, and Mueller can prove it

Trump’s lawyers say he’s above the law. They clearly don’t understand it.