SIMPLE OBSTRUCTION:

Over the years, Trump has used his Twitter account to target many different people. They now include potential witnesses in Mueller's investigation. One is Sally Yates, the fired acting attorney general who warned the White House that Flynn may be compromised and that Flynn had misled Vice President Mike Pence about his conversations with the Russian ambassador. Ahead of her testimony before a Senate Judiciary subcommittee in May, Trump suggested Yates leaked classified information to the press. He tweeted, "Ask Sally Yates, under oath, if she knows how classified information got into the newspapers soon after she explained it to W.H. Counsel."

Another is Attorney General Jeff Sessions, who was a senior adviser to the Trump campaign. On top of the interest in his own interactions with Russians, Sessions is likely an important witness in the firing of Comey and in events surrounding Comey's Valentine's Day meeting with Trump, at which Comey says Trump pressured him to drop the Flynn investigation. Trump ratcheted up the pressure on Sessions in recent weeks, describing him on Twitter as "beleaguered" and "VERY weak," and may well return to this theme if and when Sessions' involvement is back in the news.

Comey himself will be a key witness to any obstruction case. Trump tweeted on May 12 that Comey "better hope that there are no 'tapes' of our conversations before he starts leaking to the press!" While Trump didn't end up live-tweeting Comey's testimony before the Senate Intelligence Committee as he'd promised, his son Don Jr. did, disputing Comey's account and interpretation of Trump's actions.

Other potential witnesses to find themselves on the receiving end of Trump's Twitter account include former acting (and current Deputy) FBI Director Andrew McCabe - whom Trump has criticized as conflicted - and Deputy Attorney General Rod Rosenstein, who penned the letter recommending Comey's dismissal but also appointed Mueller. Trump has launched broadsides against these potential witnesses in comments to the press as well. Sessions, he told the New York Times, should not have recused. Trump tried to impugn Rosenstein's independence by saying the Deputy AG (incorrectly, as it turns out) was from Baltimore, a place with few Republicans. 

Can Trump tweet his way into criminal liability? Witness-tampering, a subspecies of obstruction, is dealt with in section 18 U.S.C. § 1512. Alongside conduct like killing or using physical force against a witness, the section outlaws "knowingly" using intimidation,  threatening a person, or corruptly persuading a person with the intent to influence, delay or prevent their testimony in an official proceeding, or to cause them to withhold their testimony.

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