The whistleblower complaint over President Trump’s call with a foreign leader concerns his communication with Ukraine, a source familiar with the complaint confirmed to The Hill on Wednesday.

The source confirmed numerous media reports that the complaint is about Trump's call with Ukrainian President Volodymyr Zelensky, in which he asked the foreign leader to investigate allegations of corruption against 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 son.

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Lawmakers reviewed the confidential complaint behind closed doors Wednesday afternoon, which comes after acting Director of National Security Joseph Maguire previously declined to share the whistleblower's disclosure.

Maguire, after consultation with the Justice Department, has argued that the allegations fell outside the intelligence community whistleblower statute. His determination runs counter to the assessment made by Inspector General Michael Atkinson, who notified Congress of the disagreement. Atkinson told the congressional Intelligence committees that he believes the complaint both falls within the statute and that it is an urgent concern.

Revelations that Trump sought to pressure Zelensky to investigate his political rival has sparked a new wave of Democrats coming out in favor of impeachment.

Their efforts to remove Trump from office also received a powerful new supporter: 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.), who announced Tuesday evening that Democrats were launching a formal impeachment inquiry into the president and his conduct.

And with the greenlight from the speaker, the House is nearing the 218 members needed in order for Trump to be removed from office.

Democrats also say their argument for impeachment was further bolstered by a readout of the call that the White House released Wednesday morning amid growing support, which showed that Trump asked Zelensky for a “favor” before asking him to investigate Biden.

While Trump has admitted to making the investigation request, he has denied withholding financial aid used to help Ukraine combat Russian aggression as leverage.

Republicans, however, say the partial transcript supports the president's claims that there was no quid pro quo, though some did express concerns about what Trump said to Zelensky in his July 25 phone call.

Congress's review of the complaint comes one day before Maguire is set to testify publicly before the House Intelligence Committee about his handling of the whistleblower complaint.