(CNN) Acting Director of National Intelligence Joseph Maguire threatened to resign if the White House tried to restrict his testimony before Congress, the Washington Post reported on Wednesday.

Citing current and former US officials familiar with the matter, the report came the day before Maguire's highly anticipated congressional testimony about a whistleblower complaint submitted to the Intelligence Community Inspector General. A source familiar with the situation previously told CNN that the complaint included a July 25 phone call between President Donald Trump and Ukrainian President Volodymyr Zelensky. During that call, Trump pushed for an investigation into potential 2020 political rival Joe Biden and his son, Hunter, a White House transcript shows.

House Speaker Nancy Pelosi cited the whistleblower's complaint in her announcement of formal impeachment proceedings Tuesday.

The officials told the Post that Maguire informed the White House that he would not omit information from his upcoming testimony.

The paper reported that the stance was partially meant to push the White House to formally decide whether to assert executive privilege regarding the complaint --- such that Maguire effectively indicated that he would cooperate with Congress unless the White House legally blocked him from doing so, the officials told the Post

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