Joseph Maguire, acting director of national intelligence, speaks during a House Intelligence Committee hearing in Washington, D.C., U.S., on Thursday, Sept. 26, 2019.

His scheduled appearance on Capitol Hill comes amid a firestorm over the release of a whistleblower complaint related to President Donald Trump's efforts to get the Ukrainian government to investigate the family of former Vice President Joe Biden, one of his chief rivals for the presidency in 2020.

Joseph Maguire, acting director of National Intelligence, vowed to step down if he was not allowed to appear before Congress, a source familiar with the matter told NBC News. The Washington Post first reported Maguire's threatened resignation , citing current and former U.S. officials.

The acting U.S. intelligence director denied reports Wednesday that he threatened to resign if the Trump administration tried to stop him from testifying freely before Congress on Thursday.

"At no time have I considered resigning my position since assuming this role on Aug. 16, 2019," Maguire said in a statement. "I have never quit anything in my life, and I am not going to start now. I am committed to leading the Intelligence Community to address the diverse and complex threats facing our nation."

White House press secretary Stephanie Grisham denied the reports Wednesday. In a tweet, she said, "This is actually not true."

In a statement, Washington Post executive editor Martin Baron said, "We stand by the story."

Trump's push to get Ukrainian President Volodymyr Zelensky to probe Biden's son, Hunter, and his administration's initial refusal to send the whistleblower complaint to Congress, prompted House Speaker Nancy Pelosi to announce an impeachment inquiry into Trump. The House Intelligence Committee is expected to receive the document Wednesday.

Maguire told the White House that he did not want to withhold information from Congress, according to the Post.

In a statement about the complaint Tuesday, Maguire said that he has "upheld [his] responsibility to follow the law every step of the way." He added that he looked forward to "continuing to work with the Administration and Congress to find a resolution regarding this important matter."

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