Mr. Pompeo has often said that the Trump administration has not been tested by a true foreign crisis and that sooner or later one was coming. Now he is caught in the middle of a domestic one . He will be pressed to explain what he knew — after acknowledging that he listened in on a July 25 phone call between Mr. Trump and the newly elected president of Ukraine, Volodymyr Zelensky — and how he reacted when he heard his boss seek political help.

“I was on the phone call,” he said Wednesday, but ignored a question about what he thought about Mr. Trump’s requests.

That admission was a marked change. In interviews in recent weeks, Mr. Pompeo repeatedly evaded questions about the content of the call between the two presidents, and never volunteered that he had listened in.

In an interview on Sept. 22, days before the White House released a transcript of the call, Mr. Pompeo suggested that he was unaware of the details, telling Martha Raddatz of ABC that he had not seen the whistle-blower report and then describing American policy toward Ukraine in traditional terms, without reference to the favors Mr. Trump sought.

The Benghazi hearings that first brought national attention to Mr. Pompeo, then a conservative congressman from Kansas, investigated systematic security failings after four Americans, including Ambassador J. Christopher Stevens, were killed in an attack on a diplomatic outpost and a nearby C.I.A. annex in 2012. The two-year congressional inquiry — one of the most bitterly partisan in history — concluded that the State Department and the C.I.A. did not appreciate the high security risk in Benghazi, but found no evidence that Mrs. Clinton was directly to blame.

Like the Benghazi hearings, the impeachment proceedings, opened last week by House Democrats, are as immersed in diplomacy as they are in political intrigue. In recent days Mr. Pompeo’s role in the Ukraine chain of events has become increasingly clearer — and ever closer to the center of the controversy.