Speaker Nancy Pelosi Nancy PelosiDemocratic senator to party: 'A little message discipline wouldn't kill us' Overnight Health Care: New wave of COVID-19 cases builds in US | Florida to lift all coronavirus restrictions on restaurants, bars | Trump stirs questions with 0 drug coupon plan Overnight Defense: Appeals court revives House lawsuit against military funding for border wall | Dems push for limits on transferring military gear to police | Lawmakers ask for IG probe into Pentagon's use of COVID-19 funds MORE (D-Calif.) said the complaint over President Trump’s call with a foreign leader “must be addressed immediately” and warned of a "new stage of investigation" if the administration blocks the whistleblower.

“This violation is about our national security. The Inspector General determined that the matter is ‘urgent’ and therefore we face an emergency that must be addressed immediately,” Pelosi wrote in a letter to members of Congress released Sunday.

The anonymous complaint, the existence of which was made public last week, reportedly includes allegations that Trump asked Ukrainian leaders to investigate former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE and his family. No evidence has emerged to suggest Biden acted with his son’s interests in mind with regard to his dealings with Ukraine.

ADVERTISEMENT

Trump denies he asked Ukraine to investigate Biden, but the White House has not released the complaint or transcript of the call.

Members of the House Intelligence Committee on Thursday received a briefing from the intelligence community's inspector general but did not receive information about the complaint's contents. Acting Director of National Intelligence Joseph Maguire has refused to provide the information to the committee.

Trump told reporters Sunday he would consider releasing the transcript of his call with Ukrainian President Volodymyr Zelensky.

“If the Administration persists in blocking this whistleblower from disclosing to Congress a serious possible breach of constitutional duties by the president, they will be entering a grave new chapter of lawlessness which will take us into a whole new stage of investigation,” Pelosi said.

Pelosi also called on congressional Republicans “to join us” in asking the acting director of national intelligence to “obey the law as we seek the truth to protect the American people and our Constitution.”

In her letter, Pelosi, who has been cautious of calling for impeachment, fails go as far as some of her fellow Democrats who have said the allegations of Trump using the presidency to bully foreign leaders into digging up information on his political opponents calls for an impeachment inquiry.

More than half of House Democrats have backed an impeachment inquiry over Trump’s alleged interference in the 2016 election, according to The Hill’s whip list.

Former special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE’s report alleged the Trump campaign welcomed Russian help during the election against his Democratic opponent Hillary Clinton Hillary Diane Rodham ClintonBloomberg rolls out M ad buy to boost Biden in Florida Hillicon Valley: Productivity, fatigue, cybersecurity emerge as top concerns amid pandemic | Facebook critics launch alternative oversight board | Google to temporarily bar election ads after polls close Trump pledges to make Juneteenth a federal holiday, designate KKK a terrorist group in pitch to Black voters MORE.

House Intelligence Committee Chairman Adam Schiff Adam Bennett SchiffSchiff to subpoena top DHS official, alleges whistleblower deposition is being stonewalled Schiff claims DHS is blocking whistleblower's access to records before testimony GOP lawmakers distance themselves from Trump comments on transfer of power MORE (D-Calif.) has been similarly cautious to back impeachment, but said Sunday that if allegations over Trump reaching out to Ukraine are true, impeachment “may be the only remedy.”

--This report was last updated on Sept. 23 at 5:30 a.m.