Recently, I explained why the US House of Representatives isn't violating President Donald Trump's "Sixth Amendment Rights."
Short version: The impeachment inquiry isn't a criminal prosecution. There are no criminal "charges" for him to be informed of, and if he is "charged" (which would happen at the end of that inquiry with a vote to impeach for X, Y, and Z), he faces no criminal penalties. Impeachment is a politicized workplace discipline process, not the Manson Family trial.
Now House Intelligence Committee Chairman Adam Schiff (D-CA) is quacking about "witness intimidation," because Trump said some mean things on Twitter about an impeachment inquiry witness, Marie Yovanovitch, the former US ambassador to Ukraine.
Um ... no. This works both ways. Trump may not have "Sixth Amendment rights," but he still has "First Amendment rights." Publicly saying mean things about a disgruntled former underling who's publicly criticizing you is free speech, not "witness intimidation."