From Los Angeles Times:

Paul Rosenzweig, who worked with independent counsel Kenneth Starr during the investigation of President Clinton that led to his impeachment in 1998, said Trump’s angry tweets about the whistleblower can be viewed as attempted witness tampering.

“He obviously doesn’t think it’s criminal, because he does it publicly,” said Rosenzweig, now a senior fellow at the R Street Institute, a nonpartisan think tank in Washington. “But the truth is, that’s wrong. Witness tampering doesn’t have to be quiet. Most people keep it quiet because it’s a crime.”