WASHINGTON — House Democrats began a potentially pivotal week on Capitol Hill Monday as frustration mounted within the caucus over the slow pace of the impeachment inquiry process against President Donald Trump. Rank-and-file members demanded the release of a recent intelligence community whistleblower complaint against the president.

The pressure on House Speaker Nancy Pelosi, D-Calif., to accelerate the methodical investigation into the president and his administration — a pace she has argued is necessary to build a case strong enough to win over public support on impeachment — has dramatically escalated over the past week, after Democrats were told that the whistleblower's complaint would not be released to Congress, as required by law.

House leadership called a members-only caucus meeting for Tuesday afternoon that is expected to tackle next steps in both the impeachment inquiry and the various congressional investigations involving Trump, multiple sources told NBC News.

On Sunday, Pelosi hinted that Democrats might expedite their investigation, writing in a letter to the caucus that they may be entering “a whole new stage.” This came after she told NPR on Friday that she had not changed her position on impeachment.

“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,” she wrote in the letter sent Sunday.

She added that the acting director of national intelligence, Joseph Maguire, would need to hand over the whistleblower’s complaint to Congress when he testifies about the matter at a public hearing before the House Intelligence Committee on Thursday.

“We also expect that he will establish a path for the whistleblower to speak directly to the House and Senate Intelligence Committees as required by law,” she said.

On Monday, Rep. David Cicilline, D-R.I., a member of House Democratic leadership and the House Judiciary Committee, said Democrats should move forward quickly with impeachment if Maguire doesn't turn over the complaint.

"That’s independent grounds to file articles of impeachment," he said in a telephone interview with NBC News Monday. "That relates to this ongoing obstruction and stonewalling."

On Sunday, Intelligence Committee Chairman Adam Schiff, D-Calif., appeared to shift his approach toward impeachment, breaking with Pelosi's cautious stance, saying that he had been "very reluctant" to go down the path — but with the president's recent actions, "we may very well have crossed the Rubicon here.”

“If the president is essentially withholding military aid [to Ukraine], at the same time that he is trying to browbeat a foreign leader into doing something illicit, that is, providing dirt on his opponent during a presidential campaign, then that may be the only remedy that is co-equal to the evil that that conduct represents,” Schiff said in an interview on CNN’s “State of the Union.”

While Congress has been blocked from learning the contents of the complaint, The New York Times and other outlets have reported that a phone call between Trump and Ukraine’s leader led to the complaint, which The Washington Post reported focuses on a "promise" Trump made.

Rep. Ro Khanna, D-Calif., noted the evolving tone from House leaders.

“There has been a shift," he said on MSNBC’s “Kasie DC” Sunday. "And I think Congressman Schiff in particular talking about impeachment is a big deal. I mean, if these allegations are true, it's a clear abuse of the president's office.”

Rep. Harley Rouda, D-Calif., noted the rising pressure within the caucus for action. “As this continues to build, as we see one activity after another with clear signs of corruption, you are going to see more and more people within the Democratic caucus come to the side of an impeachment inquiry,” he said on “Kasie DC.”

Khanna said he expected the House Judiciary Committee “will refer something to the House.” The panel is currently exploring, as part of its impeachment inquiry, whether to recommend articles of impeachment against the president to the full House.

The pressure for action from some Democratic lawmakers was rising fast as the week began. Late Saturday, Rep. Alexandria Ocasio-Cortez, D-N.Y., criticized Democratic leaders for not yet moving to impeach Trump.

“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,” she tweeted.

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. — Alexandria Ocasio-Cortez (@AOC) September 22, 2019

It wasn't the only vocal expression of frustration from within the caucus over the past several days.

On Monday, Reps. Rashida Tlaib, D-Mich., and Al Green, D-Texas, joined the youth-led group By the People to demand that the House take immediate action toward impeaching Trump.

"He is jeopardizing endangering our democracy every single day," Tlaib said at the event. "We have a whistleblower complaint that was filed by a member of the intelligence community and the director of the intelligence blocked it from being shared to the United States Congress violating the law. Enough is enough.”

Asked if Pelosi is doing enough as speaker, Green said: "The question is if Congress is doing enough.”

Rep. Pramila Jayapal, D-Wash., a co-chair of the Congressional Progressive Caucus, said Monday that Democrats are becoming more frustrated with the litigation strategy of investigating the president.

“I think you’re going to see in the caucus what I’m already seeing, there are many who are realizing that this president is going to tie things up in court. He won’t accept any lower court rulings,” Jayapal said, before adding that the court cases are taking too much time.

Jayapal, who is supportive of impeachment, said that Democrats should be wary of waiting for the next explosive thing to help make the case for impeachment. She says the president’s own words are enough to move to impeachment.

“The smoking gun is Donald Trump,” Jayapal said. “And we have to be careful about continually looking for the smoking gun. He is shooting that gun off in public almost every single day.”

Before last week’s hearing with former Trump campaign manager Corey Lewandowski, an impassioned Rep. Jackie Speier, D-Calif., told NBC News that Democrats should hold Lewandowski in contempt, and that if they are unwilling to do so, Democrats should stop investigating.

Then Friday, during a meeting of Judiciary Committee Democrats, two Democratic aides say committee leadership tried to calm members who were angry that the committee didn’t hold him in contempt on the spot during the hearing. Judiciary Committee Chairman Jerry Nadler, D-N.Y., only threatened to do so after Lewandowski stonewalled Democrats’ questions for several hours.

More than half of the House Democratic caucus has voiced support for the impeachment inquiry already underway. Nadler used that process to explain why leaders would be unable to also launch a simultaneous impeachment process against Supreme Court Justice Brett Kavanaugh, amid new allegations."We have our hands full with impeaching the president right now and that’s going to take up our limited resources and time for a while," he said in an interview last week.

Meanwhile, in the Senate on Monday, Senate Minority Leader Chuck Schumer, D-N.Y., called on Senate Majority Leader Mitch McConnell, R-Ky., and GOP committee chairmen to take immediate action to secure the whistleblower's complaint.

"The Republican Senate’s 'see no evil, hear no evil' attitude toward such a serious national security concern is unacceptable and must change," Schumer wrote in a letter to the GOP leaders, calling on them to take action.

The GOP has largely stayed silent regarding the whistleblower complaint, with several members defending the president, though Sen. Mitt Romney of Utah became the first Republican on Sunday to express grave concern about the emerging details of the complaint.

“If the President asked or pressured Ukraine’s president to investigate his political rival, either directly or through his personal attorney, it would be troubling in the extreme. Critical for the facts to come out,” he tweeted.