As the Washington Post reported late Monday that President Donald Trump ordered a hold on U.S. military aid to Ukraine just days before he pressured that country's leader to investigate former Vice President Joe Biden—a move critics described as a serious abuse of power and "extortion"—a slew of Democrats previously unsupportive of impeachment began speaking out in favor of removing the president from office.

"The dam has burst, folks," tweeted progressive advocacy group Indivisible after over a dozen House Democrats voiced support for impeachment proceedings against Trump Monday night, adding to the majority of the House Democratic caucus that already backed impeachment.

"The corruption of this administration has no bottom. The level of lawlessness is staggering. Our democracy needs defending, now more than ever. I'm ready to impeach."

—Rep. Veronica Escobar

The Post, citing three anonymous senior administration officials, reported that Trump ordered Acting Chief of Staff Mick Mulvaney to "hold back almost $400 million in military aid for Ukraine at least a week before a phone call in which Trump is said to have pressured the Ukrainian president to investigate the son of former vice president Joe Biden."

"Administration officials were instructed to tell lawmakers that the delays were part of an 'interagency process' but to give them no additional information—a pattern that continued for nearly two months, until the White House released the funds on the night of Sept. 11," according to the Post.

Some described the Post story, which was confirmed by other outlets, as a "smoking gun," but Rep. Pramila Jayapal (D-Wash.) said Monday night that Trump himself is the smoking gun and impeachment is the only solution.

"Congress appropriated the aid. The president withheld the aid because he wanted to pressure a foreign power to investigate his political opponent. The only remedy for this is impeachment. Period," tweeted Jayapal. "This is not the latest thing Trump has done. It is the latest thing we have found out about. Imagine what else he is doing? We should stop looking for a smoking gun. Trump is the smoking gun."

Congress appropriated the aid. The president withheld the aid because he wanted to pressure a foreign power to investigate his political opponent. The only remedy for this is impeachment. Period. https://t.co/ECnykgPyUE — Rep. Pramila Jayapal (@RepJayapal) September 24, 2019

About two hours before the Post revelations were published, seven so-called moderate House Democrats penned an op-ed describing Trump's alleged attempt to pressure a foreign government to investigate a political rival as "an impeachable offense."

"We call on our colleagues in Congress to consider the use of all congressional authorities available to us, including the power of 'inherent contempt' and impeachment hearings, to address these new allegations," wrote freshman Democratic Reps. Gil Cisneros (Calif.), Jason Crow (Colo.), Chrissy Houlahan (Penn.), Elaine Luria (Va.), Mikie Sherrill (N.J.), Elissa Slotkin (Mich.), and Abigail Spanberger (Va.).

"WHOA," tweeted Indivisible co-executive director Ezra Levin. "These are about the last Dems I expected to come out for impeachment. Something's changing folks."

Joining the freshman Democrats in voicing support for impeachment hearings on Monday were Reps. Debbie Dingell (Mich.), Veronica Escobar (Texas) John Larson (Conn.), Rosa DeLauro (Conn.), Dean Phillips (Minn.), and Angie Craig (Minn.).

"The corruption of this administration has no bottom," Escobar said following the Post report. "The level of lawlessness is staggering. Our democracy needs defending, now more than ever. I'm ready to impeach."

Speaking to Politico, one anonymous Democratic lawmaker described the flood of support for impeachment in just a few hours as a "seismic change" inside the House caucus.

House Speaker Nancy Pelosi (D-Calif.), who has been under pressure for months to drop her opposition to impeachment, is apparently beginning to feel the heat.

Following its story on Trump's order to withhold aid to Ukraine, the Post reported late Monday that Pelosi "has been gauging the mood of her caucus members about whether they believe that allegations that Trump pressured the Ukrainian president to investigate a political foe are a tipping point."

"She was making calls as late as Monday night," according to the Post, "and many leadership aides who once thought Trump's impeachment was unlikely now say they think it's almost inevitable."

UPDATED: Pelosi is privately talking to members abt impeaching Trump, a swath of moderate House Dems are coming out in favor of his ouster, & leadership folks think we've hit the tipping point. The watershed moment. w/@mikedebonishttps://t.co/xwbW4J7efV — Rachael Bade (@rachaelmbade) September 24, 2019

With Pelosi set to meet with lawmakers on Tuesday, progressive advocacy groups urged the public to keep up the pressure on their representatives and continue building momentum in favor of impeachment.

"It's going to be busy a day tomorrow for Congress," tweeted MoveOn. "Let's keep their phones ringing tomorrow and make impeachment happen. Call your reps. (877) 650-0039."