Pressure on House Speaker Nancy Pelosi to drop her longstanding opposition to impeachment has reached new heights following reports that President Donald Trump asked Ukraine's leader to investigate former Vice President Joe Biden, a move that appears to be at the heart of an urgent whistleblower complaint Trump's intelligence chief is refusing to turn over to Congress.

Progressive groups and members of Pelosi's House Democratic caucus have aired their frustrations with the Speaker in public in since the new reports of Trump's latest potential crime, which adds to a mountainous list of transgressions that impeachment supporters say is more than enough to remove the president from office.

"Pelosi must stop playing craven political games and lead now. It's past time for her to throw her full support behind moving forward with impeachment proceedings."

—Heidi Hess, Credo Action

"At this point, the bigger national scandal isn't the president's lawbreaking behavior—it is the Democratic Party's refusal to impeach him for it," Rep. Alexandria Ocasio-Cortez (D-N.Y.) tweeted late Saturday. "It is one thing for a sitting president to break the law. It's another to let him."

Speaking to reporters on Sunday, Trump confirmed that he discussed Biden and his son Hunter with Ukrainian President Volodymyr Zelensky but denied there was any quid pro quo.

Pelosi, in the wake of initial reports on Trump's request to the Ukrainian leader, condemned the White House for "stonewalling" Congress as it seeks to investigate the whistleblower complaint, which was reportedly filed by an intelligence official in response to the president's alarming misconduct.

Heidi Hess, co-director of Credo Action, said in a statement that "it's awfully bold of Speaker Pelosi to call out stonewalling tactics when she herself has stonewalled impeachment efforts for the past eight months."

"Donald Trump has committed numerous impeachable offenses and threatens our communities, our democracy, and our planet on a daily basis," said Hess. "Pelosi must stop playing craven political games and lead now. It's past time for her to throw her full support behind moving forward with impeachment proceedings. Doing anything less further cements her legacy as the Speaker of the House who refused to do her constitutional duty and was complicit in Trump's crimes and his bigotry."

In a Dear Colleague letter sent to Republican and Democratic lawmakers on Sunday, Pelosi warned that Trump would enter a "grave new chapter of lawlessness" if he refuses to disclose the whistleblower complaint to Congress. While Pelosi hinted at the possibility of a "whole new stage of investigation," she did not mention impeachment.

A majority of the House Democratic caucus supported impeachment proceedings against the president prior to the Ukraine revelations. While the House Judiciary Committee is holding what some Democrats have characterized as impeachment hearings, progressives say the panel's inquiry is not nearly enough.

As the Washington Post reported Sunday, tensions among House Democrats are beginning to boil over as they worry their investigations into the president are completely meaningless without the possibility of impeachment.

Two anonymous White House officials told the Post that the Trump administration is not worried about "defying or mocking" the Judiciary Committee "because Pelosi has made it clear she is not interested in impeachment."

"We back off of everything! We've been very weak," Rep. Steve Cohen (D-Tenn.), a member of the House Judiciary Committee, told the Post.

To build outside pressure on Pelosi and other Democratic opponents of impeachment, the Progressive Change Campaign Committee (PCCC) circulated a petition over the weekend denouncing the House Speaker's refusal to act in the face of Trump's illegal behavior.

"In an interview with NPR, Nancy Pelosi reiterated yet against that she will not impeach Trump. This is unacceptable, and everyone is saying so," said PCCC. "Together, we must send a resounding message that the American people are ready and demand impeachment proceedings begin now."