WASHINGTON — 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.”
The message seemed to imply that if Democrats in the House were to ever try to build an impeachment case against Mr. Trump, attempts to interfere with their work could be used as evidence.
One of the chairmen who signed the letter, Representative Elijah E. Cummings of Maryland, who leads the Oversight and Reform Committee, announced last week that Mr. Cohen would testify publicly for the first time next month about his work on behalf of Mr. Trump. The hearing promises to be a blockbuster session that could further erode Mr. Trump’s public image and clarify the extent of his legal exposure.
In August, Mr. Cohen pleaded guilty in federal court in Manhattan to tax fraud, making false statements to a bank and a campaign finance violation. He later pleaded guilty to an additional charge of lying to Congress about how long negotiations for a Trump Tower project in Moscow went on in 2016. He acknowledged that Mr. Trump’s associates pursued the project well into 2016, as the Kremlin was escalating its efforts to interfere in the American election on his behalf.
In court, he said Mr. Trump had directed him to arrange payments to two women during the 2016 campaign to stop them from speaking publicly about affairs they said they had with him. He worked closely alongside Mr. Trump during his time as a businessman and candidate, and has spent more than 70 hours with federal prosecutors in Manhattan as well as with Mr. Mueller’s team.
As Mr. Trump’s attacks on him have intensified, Mr. Cohen has told friends that he is worried for his safety, though no public evidence of any such threats has emerged.
Mr. Trump’s comments about Mr. Cohen on Saturday, made during a friendly interview with Jeanine Pirro of Fox News, were his first extended remarks on the matter since Mr. Cohen’s congressional testimony was announced. Asked by Ms. Pirro if he was “worried” about the testimony, Mr. Trump called Mr. Cohen “weak” and asserted — in contradiction to filings by federal prosecutors in Manhattan — that Mr. Cohen had “no information” on him.
“He’s in trouble on some loans and fraud and taxicabs and stuff that I know nothing about,” Mr. Trump said. “And in order to get his sentence reduced, he says, I have an idea, I’ll tell — I’ll give you some information on the president.”
Mr. Trump then implied that Mr. Cohen ought to be sharing information instead on his father-in-law, whose name he said he did not know.
“But he should give information maybe on his father-in-law, because that’s the one that people want to look at,” he said, adding, “That’s the money in the family.” Pressed by Ms. Pirro for more details, Mr. Trump said, “I don’t know, but you’ll find out, and you’ll look into it because nobody knows what’s going on over there.”
Mr. Cohen’s father-in-law, Fima Shusterman, emigrated from Ukraine in 1975. The year before his daughter’s marriage to Mr. Cohen, he pleaded guilty to evading federal reporting requirements for large cash transactions, admitting that he had cashed $5.5 million worth of checks to evade disclosure laws. He was sentenced to probation after cooperating with prosecutors in a related case.
He and Mr. Cohen have been business associates for years, particularly related to taxicabs, and have also lent millions of dollars to another Ukrainian immigrant, Semyon Shtayner, in recent years. There has been no public indication that prosecutors are pursuing Mr. Shusterman.
Saturday was not the first time Mr. Trump has targeted Mr. Cohen’s family. After Mr. Cohen pleaded guilty to his crimes, the president said Mr. Cohen had done so only to save his wife and his father-in-law.
As they prepared to indict him, prosecutors had signaled to Mr. Cohen that his wife could be implicated, since she had also signed his tax returns. That threat helped push Mr. Cohen to plead guilty to the charges, according to a person familiar with the events.
Prosecutors have made no move toward his wife since Mr. Cohen’s guilty plea in August.
The statement from the three chairmen was also signed by Representative Adam B. Schiff of California, the chairman of the Intelligence Committee, and Representative Jerrold Nadler of New York, the Judiciary Committee chairman. Both men are investigating the president for various reasons, and Mr. Nadler’s committee would be the body to weigh a possible impeachment.
Mr. Nadler has said he will wait for Mr. Mueller to complete his work before seriously considering impeachment, but he told The New York Times last month that the campaign finance fraud laid out by prosecutors in New York was likely to meet the criteria for an impeachable offense and warranted congressional investigation.
He has also issued repeated warnings about Mr. Trump’s attacks on federal law enforcement officials, including Mr. Mueller.
In 1974, the Judiciary Committee included obstruction of congressional investigations and proceedings among a litany of offenses for which it voted to impeach President Richard M. Nixon. He resigned from office before the full House voted on the impeachment articles.