Mark F. Pomerantz, a former prosecutor who once helped lead an investigation of Donald J. Trump, appeared on Friday before the Republican-led House Judiciary Committee for a deposition but declined to answer many of its questions about the prosecution of the former president on charges of falsifying business records.
Mr. Pomerantz cited the confidentiality of the pending case and invoked a range of privileges, including the Fifth Amendment right against self-incrimination, to avoid answering the committee’s questions, according to an opening statement obtained by The New York Times. He said he had agreed to appear because he respected the rule of law.
“What I do not respect is the use of the committee’s subpoena power to compel me to participate in an act of political theater,” Mr. Pomerantz added. “We are gathered here because Donald Trump’s supporters would like to use these proceedings to attempt to obstruct and undermine the criminal case pending against him, and to harass, intimidate and discredit anyone who investigates or charges him.”
Mr. Pomerantz repeatedly cited New York law protecting the privacy of active investigations and said he had been threatened with possible prosecution for violating the secrecy of the grand jury process if he answered certain questions.
“While I am certain I broke no laws, I am not required to answer questions if my answers might be used against me in a criminal prosecution,” he said, adding: “It gives me no joy to invoke my legal rights, but I am glad that the law allows me not to cooperate with this performance of political theater.”
Forcing Mr. Pomerantz’s appearance was a victory for the Republicans who have rushed to use their power in the House to defend Mr. Trump since it became clear that Alvin L. Bragg, the Manhattan district attorney, was poised to charge the former president.
Mr. Bragg, who has railed against Republican efforts to interfere with his work, sued to try to block the Judiciary Committee — which is led by Representative Jim Jordan, Republican of Ohio and a close ally of Mr. Trump — from compelling Mr. Pomerantz to testify. But that effort failed.
A representative for Mr. Bragg appeared with Mr. Pomerantz in Washington on Friday.
“The district attorney’s office is participating in today’s deposition and asserting our rights to oppose disclosure of confidential information protected by law,” a spokeswoman for Mr. Bragg said in a statement.
A federal judge ruled last month that congressional Republicans on the Judiciary Committee had a constitutional right in their role as legislators to question Mr. Pomerantz.
“It is not the role of the federal judiciary to dictate what legislation Congress may consider or how it should conduct its deliberations,” the judge wrote, adding: “Mr. Pomerantz must appear for the congressional deposition. No one is above the law.”
The Judiciary Committee quoted from that ruling on Twitter on Friday as Mr. Pomerantz was testifying.
Even so, some Republicans expressed frustration at Mr. Pomerantz’s refusal to answer questions.
“He has answered no questions,” Representative Darrell Issa, Republican of California and a member of the committee, told reporters outside the deposition. “He has asserted, on behalf of the D.A.’s office and himself what I would characterize as taking the Fifth — effectively taking the Fifth every single time. I’ve never had a more obstructive and less cooperative witness in my over 20 years in Congress.”
Mr. Trump is facing prosecution in New York over his role in a hush-money payoff to an adult film entertainer, Stormy Daniels, who then agreed to keep quiet about her story of an affair with him.
The 34-count indictment accused him of falsifying business records related to the payment, which was in the last days of the 2016 presidential campaign and might have functioned as an illegal donation to his candidacy. He has pleaded not guilty.
Jonah E. Bromwich contributed reporting.
