The official did not identify the name of the rival candidate. Separately, a senior administration official told Fox News the White House has been working as quickly as it can to release to Congress the whistleblower complaint involving President Trump's conversations with the leader of Ukraine, as long as it's legally possible.

The news came just hours after House Speaker Nancy Pelosi initiated a formal impeachment inquiry by alleging that the administration was hiding the complaint.

The senior administration official told Fox News that the White House had nothing to hide, that there has been no wrongdoing, and that the White House's general position has been that it will make everything possible available to Congress or the public regarding Trump's conversations with Ukrainian President Volodymyr Zelensky and the complaint to the intelligence community's inspector general.

A source familiar with the matter told Fox News this week that the whistleblower had no firsthand knowledge of Trump's July call with Zelensky. Trump vowed earlier Tuesday to release a "complete" transcript of the call by Wednesday.

A senior administration official told Fox News there are a “few words” in the transcript that will raise eyebrows, but it is nowhere near as inflammatory as Democrats have suggested.

The contents of the call, as well as the whistleblower complaint, could throw cold water on Democrats' explosive suggestions that the president improperly threatened to withhold aid to Ukraine unless it investigated Joe Biden and his son Hunter. Republicans had predicted over the weekend that such an impeachment inquiry could backfire on Pelosi, and administration officials have said Trump was concerned only with broader corruption in Ukraine.

Joe Biden has acknowledged on camera that, when he was vice president, he successfully pressured Ukraine to fire its top prosecutor, Viktor Shokin, while Shokin was investigating the natural gas firm Burisma Holdings — where Hunter Biden was on the board. Shokin himself had separately been accused of corruption.

The House speaker specifically charged that the administration had violated the law by failing to turn over the whistleblower complaint. Citing testimony that the director of national intelligence was blocking the release of that complaint, she said: "This is a violation of law. The law is unequivocal."

Fox News also was told the White House has taken steps to alleviate concerns about the precedential nature of releasing a transcript of the phone call with Zelensky. With the release of the transcript, the White House has put in place protections to preserve the confidential nature of conversations between the president and world leaders.

And, The New York Times reported that the White House had dropped its objection to the whistleblower speaking to Congress. That came after the GOP-controlled Senate passed a unanimous resolution seeking access to the whistleblower's complaint.

DNI Inspector General Michael Atkinson said in a Sep. 9 letter to the House Intelligence Committee that the whistleblower complaint "appeared credible" and related to an "urgent" matter. But the DNI general counsel said days later that, after consulting with the DOJ, the matter did not meet the legal definition of an “urgent concern," and was not subject to mandatory disclosure to Congress.

“Furthermore, because the complaint involves confidential and potentially privileged communications by persons outside the Intelligence Community, the DNI lacks unilateral authority to transmit such materials to the intelligence committees,”  Jason Klitenic, the DNI general counsel, wrote.

Acting Director of National Intelligence Joseph Maguire will testify before the House Intelligence Committee at an open hearing on Thursday.

House Intelligence Committee Chairman Adam Schiff, D-Calif., said earlier Tuesday afternoon that testimony from the whistleblower might also be imminent.

"We have been informed by the whistleblower’s counsel that their client would like to speak to our committee and has requested guidance from the Acting DNI [Director of National Intelligence] as to how to do so," Schiff said in a tweet. "We‘re in touch with counsel and look forward to the whistleblower’s testimony as soon as this week."

Despite the apparent progress in releasing the relevant information, Pelosi, D-Calif., told the nation that "the president must be held accountable" for his "betrayal of his oath of office, betrayal of our national security, and the betrayal of the integrity of our elections."

"This week, the president has admitted to asking the president of Ukraine to take actions which would benefit him politically," Pelosi said. "Therefore, today, I'm announcing the House of Representatives is moving forward with an official impeachment inquiry. I'm directing our six committees to proceed with their investigations under that umbrella.

"The president must be held accountable," she continued. "No one is above the law."

The House committees then would gather evidence and president it to Pelosi and House Judiciary Committee Chairman Jerry Nadler, D-N.Y., who would make the ultimate determination on whether to hold an impeachment vote.

A Democratic source told Fox News that the House Intelligence Committee, led by Schiff, would take the lead in the impeachment investigation -- and that Nadler might not be pleased with the arrangement.

"I don’t think he’s happy that he has less influence here, but everybody is on the same page and has the same role," the source said of Nadler.

Schiff "will have a heavy role here because the foundation of the Ukraine allegations is thorough his committee," the source said.

The speaker effectively endorsed the process, which to some degree has already been underway, after facing fresh pressure from inside the caucus to act. The move could help Democrats' disputed arguments in court that impeachment proceedings were in fact in progress, which could entitle Congress to obtain additional documents.

At the same time, at an event Tuesday, Pelosi intimated that impeachment would remain on the table, regardless of what the transcript showed. Many conservatives charged that she was moving the goalposts and lowering expectations.

"We have many other, shall we say, candidates for impeachable offenses in terms of the Constitution, but this one is the most understandable by the public," Pelosi said, referring to the Ukraine phone call allegation. "It's really important to know this: There is no requirement that there be a quid-pro-quo in the conversation."

Other prominent Democrats also seemingly said Trump should be impeached no matter what.

"The president has committed several impeachable offenses," Rep. Alexandria Ocasio-Cortez, D-N.Y., told reporters after Pelosi's remarks on impeachment. In another indication that Democrats were apparently hedging their bets on the Ukraine matter, Ocasio-Cortez said alleged Emoluments Clause violations by the president could be included in prospective articles of impeachment.

Republicans said the move would prove to be a major political mistake.

"It is a colossal error," Texas Republican Sen. John Cornyn told Fox News just prior to Pelosi's comments. "And, I’m kind of surprised that Speaker Pelosi, as shrewd as she is, would let it get to this point."

Swing district Rep. Mikie Sherrill, D-N.J., acknowledged to Fox News that supporting the impeachment inquiry "could" affect her electorally, but she maintained that Trump voters in her district "understand," and that Trump crossed a red line.

Trump, for his part, ripped into Democrats in a series of tweets immediately after Pelosi's comments, writing that "PRESIDENTIAL HARASSMENT" was in progress again.

In remarks to reporters at the United Nations on Monday, Trump denied linking the aid money to Ukraine's investigative actions. “No, I didn’t — I didn’t do it,” Trump said. But, he also repeatedly called the Bidens' actions in Ukraine a "disgrace," acknowledged that Biden had come up during the call and added: "It's very important to talk about corruption. ... Why would you give money to a country that you think is corrupt?"

House Oversight Committee Ranking Member Jim Jordan, R-Ohio, called Democrats' efforts predictable and destructive in his own fiery statement.

"Democrats have been trying to impeach the President since the beginning of this Congress," Jordan said. "Michael Cohen's testimony was a bust. John Dean's testimony was a waste of time. The Mueller report did not live up to the hype.

"Speaker Pelosi's decision to pursue impeachment now - on the basis of unsubstantiated, indirect, and anonymous allegations - only shows that the Speaker has finally succumbed to unrelenting pressure from the socialist wing of the Democrat Party," Jordan added. "This was never about Russian collusion or Ukrainian prosecutions. It is all about undoing the 2016 election and the will of the American people."

Trump is set to meet with Zelensky in New York on Wednesday. The visit was previously scheduled, unrelated to the whistleblower allegation, although the two leaders are expected to face questions about the matter from reporters.