EXCLUSIVE: A senior Trump administration official told Fox News late Tuesday that the administration will release a document showing the intelligence community inspector general found the whistleblower who leveled an explosive accusation against President Trump concerning his talks with Ukraine had indications of “political bias” in favor of “a rival candidate” of the president.

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.

Intelligence Community 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.

News of the whistleblower's possible bias came just hours after House Speaker Nancy Pelosi initiated a formal impeachment inquiry by alleging that the administration was hiding the complaint. Other top Democrats had previously said such an inquiry was already underway.

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.

Just after midnight Wednesday, Trump attorney Rudy Giuliani -- who has long publicly called for Ukraine to investigate Biden's dealings in Ukraine -- posted a series of messages on Twitter suggesting Democrats have a bigger problem on their hands.

"Democrat party is covering up a pattern of corruption involving high level members of the Obama cabinet," Giuliani wrote. "The multi-million and billion dollar pay-for-play is mind boggling. Biden Family sale of office to Ukraine was not the only one or the most egregious. Slimy Joe is not alone."

Giuliani added: "We know corrupt Ukrainian oligarch laundered $3 million to the Biden Family. But $3 to $4m more was laundered to Biden. So release all the financial records of all businesses involving Biden, Kerry’s stepson and notorious mobster Whitey Bulger’s nephew. ... Biden should agree to release records to see if he flew Hunter to China in Dec. 2013 on AF 2 to facilitate Hunter’s sale of his office to China for a total of $1.5 billion. Is there any doubt that China paid it to compromise VP. But they bought another pol as well. Guess?"

In her televised remarks, Pelosi 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."

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Meanwhile, attention focused anew Tuesday night on previous apparent efforts by Democrats to pressure Ukraine on its investigations. The Washington Post's Marc Thiessen pointed out that CNN reported in May that Democratic Sens. Robert Menendez, Dick Durbin, and Patrick Leahy pushed Ukraine’s top prosecutor not to close four investigations perceived as critical to then-Special Counsel Robert Mueller's Russia probe -- and seemingly threatened that their support for U.S. aid to Ukraine was at stake.

The senators wrote: "In four short years, Ukraine has made significant progress in building [democratic] institutions despite ongoing military, economic, and political pressure from Moscow. We have supported [the] capacity-building process and are disappointed that some in Kyiv appear to have cast aside these [democratic] principles to avoid the ire of President Trump." The senators called for the top prosecutor to “reverse course and halt any efforts to impede cooperation with this important investigation.”

It remained unclear late Tuesday exactly what the president said on his call with Ukraine's leader. Fox News is 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 upcoming planned release of the transcript on Wednesday, the White House has put in place protections to preserve the confidential nature of conversations between the president and world leaders.

Also in the evening, 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.

Acting Director of National Intelligence Joseph Maguire will testify before the House Intelligence Committee at an open hearing on Thursday, and is expected to face a series of questions about these and other issues.

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."

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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 present 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.

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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."

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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?"

"It is a colossal error." — Texas Republican Sen. John Cornyn

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.

A total of 172 House Democrats have now signaled strong support for an impeachment inquiry -- 235 Democrats and 198 Republicans are in the House, with one pro-impeachment independent. A majority would be required to successfully impeach the president. A highly unlikely two-thirds vote in the GOP-controlled Senate would be needed to convict and remove the president.

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Vice President Mike Pence would then take office in that scenario.

"The ironic thing is is that everything that our critics in the media are leveling at the president from this phone call, and leveling at our administration, everything that Democrats on Capitol Hill are running off and describing -- Vice President Joe Biden bragged about -- which was a quid-pro-quo -- withholding American aid in exchange for a specific action," Pence told Fox News' "Hannity" on Monday.

Fox News' Chad Pergram, Brooke Singman, Ronn Blitzer and Jake Gibson contributed to this report.