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On Monday morning, the president took to Twitter to praise Roger Stone, his longtime political adviser and associate, for refusing to speak to Special Counsel Robert Mueller’s team.

Mueller’s investigation into Russian election interference and Trump has zeroed in on Stone as a possible tie between Russia and the Trump campaign, investigating whether he had advance knowledge that WikiLeaks would publish thousands of emails stolen from Democrats by Russian hackers. On Sunday’s This Week on ABC, Stone vowed not to testify against the president. “Nice to know that some people still have ‘guts!'” Trump tweeted in response.

“I will never testify against Trump.” 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!” — Donald J. Trump (@realDonaldTrump) December 3, 2018

A number of legal experts from both sides of the aisle have said the president’s tweet could represent an illegal attempt to influence a witness in the special counsel’s investigation. George Conway, a longtime conservative lawyer and vocal Trump critic who is married to presidential counselor Kellyanne Conway, suggested the message violated portions of the US code governing witness tampering and obstruction of justice:

File under “Stating the obvious” — George Conway (@gtconway3d) December 3, 2018

Neal Katyal, who served as acting solicitor general under President Barack Obama, piled on, as did lawyer and ethics expert Norm Eisen.

George is right. This is genuinely looking like witness tampering. DOJ (at least with a nonfake AG) prosecutes cases like these all the time. The fact it's done out in the open is no defense. Trump is genuinely melting down, and no good lawyer can represent him under these circs https://t.co/zqFUoQvWTf — Neal Katyal (@neal_katyal) December 3, 2018

So did Obama’s former head speechwriter, Jon Favreau, alongside other prominent lawyers:

This doesn’t seem very cool or very legal. https://t.co/VHnQ5CnywE — Jon Favreau (@jonfavs) December 3, 2018