Democrats were once prepared to take extraordinary steps to preserve the whistleblower’s identity under questioning, considering him central to their investigation. But over the past month, they have grown cold to the idea of exposing him to additional scrutiny after several witnesses described how Trump leveraged access and military aid to secure a promise from Ukraine to launch investigations that could help his 2020 reelection bid.

“I think it’s quite clear we have a surfeit of evidence that corroborates in full every aspect of what happened and the policy they were pursuing,” said Rep. Gerald E. Connolly (D-Va.), a member of the Oversight and Foreign Affairs committees.

A person familiar with the discussions between the whistleblower and House investigators, speaking on the condition of anonymity to discuss private talks, said Thursday that there are no active efforts to arrange for the individual’s testimony. A spokesman for House Intelligence Committee Chairman Adam B. Schiff (D-Calif.), who is leading the inquiry, declined to comment.