After the White House said it would invoke a legal claim of absolute immunity to block Mr. Kupperman’s testimony, Mr. Kupperman filed a lawsuit, asking a federal judge to rule on the matter. Other senior White House officials could follow suit.

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In a terse letter sent Saturday to Mr. Kupperman’s lawyers, Democrats said the House did not recognize the White House’s legal claims as legitimate. They ordered Mr. Kupperman to testify on Monday or risk adding to evidence that Mr. Trump is obstructing Congress and should be held in contempt.

Mr. Trump continued on Saturday to rail against the inquiry, dismissing it on Twitter as “just as Corrupt and Fake as all of the other garbage that went on before it.” He also claimed that Speaker Nancy Pelosi, the Democratic leader who initiated the impeachment inquiry in September, was neglecting her San Francisco district, which he described as a hellscape of environmental and sanitary violations.

Mr. Reeker’s testimony on Saturday was the latest by roughly a dozen witnesses, many of whom were ordered by the administration not to cooperate but confronted with subpoenas complied anyway.

Mr. Reeker took charge of the State Department’s activities in Europe and Eurasia this March, dividing his time between Washington and Europe. A 27-year veteran of the foreign service, he had been posted around the world, including in Iraq, Germany, Italy, Hungary and Macedonia. Friends described him as a consummate diplomat, who, as one put it, “never says any word more or any word less than he should.”

Mr. Reeker testified that it was immediately clear to him that President Trump had been relying on two other officials to carry out his policy toward Ukraine: Gordon D. Sondland, a Republican fund-raiser who had been named ambassador to the European Union, and Kurt D. Volker, the special envoy to Ukraine. Mr. Sondland was one of seven ambassadors who reported to Mr. Reeker.

Mr. Reeker also testified that he understood that Ms. Yovanovitch, the ambassador in Kiev and a longtime colleague, was under attack by Mr. Giuliani and his allies. Among their accusations, the ambassador’s critics claimed that she was disloyal to the president — charges that Mr. Reeker considered completely spurious.