Acting Director of National Intelligence Joseph Maguire is expected to testify at the House Intelligence Committee in an open hearing on Thursday, when Pelosi demands he turn over a whistleblower's full complaint and "establish a path for the whistleblower to speak directly to the House and Senate Intelligence Committees as required by law."
"We must be sure that the President and his Administration are always conducting our national security and foreign policy in the best interest of the American people, not the President's personal or political interest," Pelosi wrote in a Dear Colleague letter on Sunday. "This violation is about our national security."
Several House Democrats who have been reluctant to sign onto the impeachment effort signaled that their patience is wearing thin Monday. Freshman Democratic Rep. Angie Craig of Minnesota has heard enough and is now openly calling on Democratic leaders to open "impeachment proceedings."
"It is clear that the sitting president of the United States placed his own personal interests above the national security of the United States," Craig, D-Minn., wrote in a statement. "We must safeguard our electoral process and our very democracy from outside threats."
Reps. Gil Cisneros of California, Jason Crow of Colorado, Chrissy Houlahan of Pennsylvania, Elaine Luria of Virginia, Mikie Sherrill of New Jersey, Elissa Slotkin of Michigan and Abigail Spanberger of Virginia -- all freshman Democratic lawmakers -- wrote in a Washington Post op-Ed Monday that if the reported whistleblower allegations are in fact true, that "we believe these actions represent an impeachable offense."
"We do not arrive at this conclusion lightly, and 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, find the truth and protect our national security,” they wrote.
These Democrats are some of the key members Pelosi would have to have on board to launch impeachment proceedings - her prior reluctance has been part of an effort to help them defend their seats next year. If they end up supporting impeachment, that changes her calculus.
This gives Pelosi, who has been calling members and gauging her caucus, a permission slip and cover to go as far publicly as they have: to seriously consider formal impeachment proceedings.
House Democrats will hold a members-only meeting at the Capitol Tuesday afternoon to discuss next steps, a Democratic leadership aide told ABC News.
Sen. Chris Murphy, who recently met with Ukraine President Zelensky, released a statement calling on the House of Representatives to begin an impeachment inquiry into "the president’s corrupt efforts to press a foreign nation into the service of his reelection campaign."
"Up until these recent developments, I had resisted calling for the House to begin impeachment proceedings, choosing instead to allow the House to consider its options free from senatorial advice. But circumstances have changed, and the seriousness of the moment requires all of us to speak out in order to preserve our nation’s commitment to the rule of law," Murphy said.
Rep. Dean Phillips, a freshman Democrat from Minnesota who does not yet publicly support an impeachment inquiry, indicated that the showdown over the whistleblower could move him into the pro-impeachment camp unless the administration begins to cooperate with congressional oversight efforts.
"It appears that our President encouraged the leader of Ukraine to investigate his political opponent, thereby inviting foreign interference in our democracy. This continues a pattern of behavior that is corrupt at best, treasonous at worst, and puts our rule of law at risk," Phillips noted. "If the reports are corroborated, we must pursue articles of impeachment and report them to the full House of Representatives for immediate consideration."
Connecticut Democratic Rep. Joe Courtney stressed that the law requires that if the inspector general determines that a whistleblower complaint meets the standard of "credible" and of "urgent concern," then the Director of National Intelligence "shall" forward the material to the House and Senate intelligence oversight committees.
"We have arrived at a critical juncture in time for the rule of law in the United States, and for this administration's faithfulness to our Constitution," Courtney stated. "This administration now finds itself at a decision point concerning the rule of law, and their adherence to it. The president and acting DNI Maguire have two simple choices when he comes before Congress later this week: follow the law and transmit the complaint to the bipartisan intelligence committees or flagrantly disobey the law and force the Congress to act to uphold the rule of law."
Senate Minority Leader Chuck Schumer called on Senate Majority Leader Mitch McConnell to take immediate action on the confidential complaint, writing McConnell to present several options, including holding hearings with Maguire, issuing a subpoena for the complaint and calling for the release of the Trump-Ukraine phone call transcript.
"This is a whistleblower complaint that has been labeled 'urgent' and 'credible' not by Democrats, but by a senior-level Trump appointee," Schumer, D-N.Y., noted. "It is the Senate's duty to take this national security matter seriously and to take action now."
At least 137 House Democrats, or about 58 percent of Pelosi's caucus, have signaled support for an impeachment inquiry, according to ABC News' records. Nevertheless, President Trump maintained that he's "not at all" worried about impeachment.
Addressing the controversy, McConnell touted his own efforts to secure security assistance funding to Ukraine, and chastised Pelosi, Schumer, and House Intelligence Committee chairman Adam Schiff for "politicizing" the work of the Intelligence committee.
"For my part, I was very glad to see the White House release security assistance funds for Ukraine," McConnell said in a floor speech Monday afternoon, adding the specific subject of the whistleblower's complaint "is still unknown."
McConnell ticked through his actions, including urging the secretaries of State and Defense to do whatever they could to bolster Ukraine against Russian aggression, but he did not criticize the Trump administration for slow-walking that $250 million in security assistance funding to Ukraine for months prior to the early-September decision to ultimately release it.
Russia poses a significant threat to U.S. interests, McConnell acknowledged, and the best way to counter it is to "rebuild our defenses, work closely with our allies and partners, and improve the capacity of those threatened by Moscow to defend themselves."
Three pro-impeachment Democrats joined a small rally at the Capitol where activists presented the case for impeachment and implored lawmakers to take action.
"We're at a point where people who probably don't want to do [impeachment] will find themselves moving in the direction of doing it simply because there's just too much evidence to ignore," Rep. Al Green, an early proponent of impeachment, told reporters Monday afternoon. "I think this is going to go to the point of the president being impeached."
Rep. Rashida Tlaib, a freshman Democrat who gained notoriety after taking office for proclaiming she would "impeach the motherf*****" called on Democratic leaders in Congress "to listen to the people and the fact that this is the right thing to do and that we have to put our country first."
"More and more people support accountability -- even those that had supported this president are now backing down and saying we have a situation that is hurting our democracy, that's jeopardizing really I think our way forward and our country and the fact that he doesn't abide by the United States Constitution," Tlaib, D-Mich., told ABC News senior congressional correspondent Mary Bruce.
Schiff, who has banded together with Pelosi in pursuit of an investigative track to build a case for impeachment, said that with the president publicly seeking foreign assistance to investigate his political adversaries, "then he may force us to go down this road."
"I have spoken with a number of my colleagues over the last week, and this seems different in kind. And we may very well have crossed the Rubicon here," Schiff, D-Calif., said on CNN's State of the Union on Sunday. "Now, we cannot afford to play rope-a-dope in the court for weeks or months on end. We need an answer. If there's a fire burning, it needs to be put out. And that's why we're going to have to look at every remedy."
ABC News' Sarah Kolinovsky and Benjamin Siegel contributed to this report.