As their fears mounted, the report said some senior White House officials secretly tried to put off the phone call with Zelensky because they were worried Trump would use it to press the case for investigating Biden and Hunter, over their dealings in Ukraine.

The phone call is now at the center of an explosive and unprecedented whistleblower complaint that a member of US intelligence filed against Trump in August, according to multiple media reports.

Read more: Trump tried to negotiate with Pelosi on the whistleblower complaint after she announced an impeachment inquiry. Pelosi told him to take a hike.

The intelligence community inspector general, Michael Atkinson, determined after a preliminary review that the complaint was "credible" and "urgent," which should have triggered a federal statute requiring the Office of the Director of National Intelligence to turn the complaint over to congressional intelligence committees.

But the acting director of national intelligence, Joseph Maguire, overruled Atkinson after consulting with the Justice Department and determined that the complaint did not meet the definition of "urgent concern" under the law because it related to an individual who is outside the DNI's purview. The decision was backed by White House counsel Pat Cipollone.

But the tide appears to have changed since House Speaker Nancy Pelosi announced a formal impeachment inquiry against Trump on Tuesday, because Politico reported afterward that the White House is now preparing to release the complaint to Congress by the end of the week. Trump also announced earlier on Tuesday that he would release a full transcript of his phone call with Zelensky.

The shift indicates the seriousness with which the White House apparently views the impeachment proceedings, even as Trump decries them as a continuation of the "witch hunt" against him and his presidency. The White House's decision also came after the Senate unanimously voted in favor of a resolution calling for the complaint to be turned over to congressional intelligence committees.

House Democrats have long been at war over whether to impeach Trump. But the unprecedented nature of his alleged request to Zelensky — asking a foreign country to manufacture dirt on a political opponent ahead of an election — set off alarms among Democrats and the national-security apparatus, and dozens of House Democrats came out in favor of impeachment in recent days as details continued to spill out about the phone call.

The timing of Trump's alleged request also raised eyebrows. Specifically, Trump slammed the brakes on a nearly $400 million military aid package to Ukraine roughly one week before the phone call, The Washington Post reported. He released the aid on September 12, one day after House Intelligence Committee Chairman Adam Schiff requested a full, unredacted version of the whistleblower's complaint from Maguire.