In a letter to House Democrats, Speaker Nancy Pelosi announced that the House will take its first formal vote on Thursday to “affirm” the impeachment investigation. The resolution they will vote on also outlines procedures for holding public hearings and releasing interview transcripts.

Republicans have long criticized Democrats for not formally authorizing the proceedings, using the issue to justify their lack of cooperation. Ms. Pelosi had insisted a vote was not necessary, making her announcement today a precautionary step. She said the measure would “eliminate any doubt” as to whether the White House was required to comply.

Representative Adam Schiff, the House Intelligence Committee chairman leading the investigation, said that House Democrats will not ask federal courts to compel testimony from uncooperative witnesses. Mr. Schiff said petitioning the courts would “allow the White House to engage us in a lengthy game of rope-a-dope.”

Charles Kupperman, the former deputy national security adviser, defied a House subpoena today by not showing up for scheduled testimony. The White House claims he is immune from testifying, and Mr. Kupperman has filed a lawsuit seeking to clarify whether he should testify.

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A White House official speaks up

The Times has obtained a copy of the opening statement that Lt. Col. Alexander Vindman, the top Ukraine expert on the National Security Council, plans to deliver to House impeachment investigators on Tuesday. Here are four highlights:

1. He was on the July 25 phone call. Colonel Vindman was listening in when President Trump spoke with the Ukrainian president, Volodymyr Zelensky — a call that kick-started the investigation. He said he was alarmed by Mr. Trump’s request that Mr. Zelensky investigate the president’s political rivals.

2. He raised concerns about Ukraine. On two occasions, Colonel Vindman went to the top lawyer at the National Security Council with concerns about the administration’s conduct with Ukraine. The first was on July 10, when he said Gordon Sondland, the ambassador to the E.U., urged Ukrainian officials to assist with the president’s political priorities. The second was after the phone call on July 25.

3. He tried to get military aid restored. In August, at the direction of his superiors at the National Security Council — including John Bolton, then the president’s national security adviser — Colonel Vindman drafted a memo to restart security aid that had been withheld from Ukraine. Mr. Trump refused to sign it.