Stop publicly intimidating witnesses, Mr President. You may not know this but it's illegal.

Three newly empowered Democratic House committee chairmen, alarmed by statements over the weekend by President Trump about his former lawyer’s planned testimony before Congress, cautioned on Sunday that any effort to discourage or influence a witness’s testimony could be construed as a crime.

The warning, a stark and unusual message from some of Congress’s most influential Democrats, underscores the increasing legal and political peril facing Mr. Trump. Democrats are beginning their own investigations of him as the special counsel, Robert S. Mueller III, appears to move toward a conclusion in his investigation into the Trump campaign’s ties to Russia and potential obstruction of justice by Mr. Trump.

In a Fox News interview on Saturday night, Mr. Trump accused the former lawyer, Michael D. Cohen, of lying about him to win leniency from federal prosecutors and spoke cryptically of the existence of damaging information against Mr. Cohen’s father-in-law. Mr. Cohen, who has been sentenced to three years in prison, has accused Mr. Trump of directing him to make illegal hush payments during the campaign.

“Our nation’s laws prohibit efforts to discourage, intimidate or otherwise pressure a witness not to provide testimony to Congress,” the chairmen wrote. “The president should make no statement or take any action to obstruct Congress’s independent oversight and investigative efforts, including by seeking to discourage any witness from testifying in response to a duly authorized request from Congress.”