(CNN) House Democrats are nearing a "tipping point" on impeachment in the wake of the Trump administration blocking a whistleblower complaint's release to Congress, with moderate Democrats showing new openness to moving ahead and House Speaker Nancy Pelosi offering a fresh sign she's easing from her resistance.

Pelosi, who sharpened her rhetoric over the weekend , is still not committing to moving forward with impeachment proceedings -- but she is feeling the tide turn.

In a brief interview with CNN as she flew from New York to Washington Monday, Pelosi again declined to say whether she would fully endorse initiating an impeachment inquiry when she meets with committee chairmen and members of the Democratic caucus on Tuesday. But she left little doubt the developments around the whistleblower's complaint had dramatically escalated the standoff with President Donald Trump and a move toward impeachment proceedings was all but certain.

"We will have no choice," Pelosi said of ultimately initiating an impeachment inquiry. She did not say whether the move toward impeachment could be slowed if the White House provided the whistleblower's report.

During the flight, Pelosi read a Washington Post op-ed from seven freshman Democrats who made the case for impeachment.

She said she had advance notice the lawmakers were planning to join forces to collectively argue the president's actions "represent an impeachable offense."

"It will be a big week," Pelosi said.

The reason: The substance of the complaint is not yet clear. And it's still unclear how the Trump administration will respond.

Multiple Democratic sources told CNN that if the White House completely stonewalls Democrats for the information, it will make the calculus easier for moving forward with impeachment proceedings. But if the White House provides a transcript, or seeks to muddy the waters in its response, that could make it more challenging for Democrats to unify behind impeachment, even after President Donald Trump's admission of discussing Vice President Joe Biden and his son in his phone call with the Ukrainian leader.

That's why this week is critical for House Democrats to decide which course to take. They are awaiting to see how the acting director of national intelligence, Joseph Maguire, responds during Thursday's high-stakes hearing before the House Intelligence Committee -- and whether the administration provides any more details about Trump's conversations with Ukrainian leaders or the substance of the whistleblower complaint.

Plus, Pelosi and House Democratic leaders are waiting to see how members respond -- particularly ones in difficult races who represent GOP-leaning districts -- to see if it's time to move ahead. Pelosi, a savvy vote counter, has no desire to move forward with impeachment if it lacks 218 votes in the House, Democratic sources say, and before the Ukraine controversy, it is almost certain that the House Democrats would have fallen well short of that majority threshold. That could change depending on how the latest controversy shakes out.

The House Democratic caucus will meet Tuesday afternoon, according to three Democratic sources. A leadership aide told CNN that the topic is likely to be about the next steps on investigations.

"The dominant focus of the caucus on Wednesday is trade, a second caucus has been added to ensure adequate time for member discussion on the whistleblower matter and a number of other pressing matters on Tuesday afternoon," a Democratic leadership aide said in a statement Monday evening.

'When are Democrats going to get tough?'

The impeachment fight remains a divisive issue in the party as Democratic leaders continue their investigations into the Trump administration -- including a new subpoena threat from a trio of House chairmen -- following a steady stream of developments on the issue over the past couple days.

Democrats, including freshmen from Trump districts and moderates who have resisted calls for impeachment, are telling their colleagues privately they are prepared to announce their support for impeachment proceedings if the controversy continues to grow -- namely if there's evidence that Trump sought to withhold military aid to Ukraine in exchange for investigating the Bidens, according to Democratic sources involved in the conversations.

That could change the calculus for Pelosi, who has in part resisted moving forward on impeachment because she has sought to protect vulnerable Democrats in GOP-leaning districts who could face voter backlash over impeachment. But if those members come out in support of impeachment proceedings, Pelosi is bound to shift her positioning as well, according to Democrats close to the speaker.

"I think we're reaching a tipping point both within our base and within our caucus," Rep. Gerry Connolly, a Democrat on the House Oversight and Foreign Affairs committees, told CNN on Monday.

"This weekend, all I hear at home is when are Democrats going to get tough?" Connolly said. "We are looking weak."

So far, as many as 137 House Democrats have publicly stated their support for an impeachment inquiry into Trump, according a CNN count -- more than half the caucus and with dozens having joined in the past two months. But that number is still far below a majority of House members and Pelosi has long warned of the political consequences of taking such a step against the Presidents.

Speaking to CNN's Poppy Harlow on "Newsroom" Monday, Democratic Rep. Vicente Gonzalez of Texas said if Trump withheld military aid to Ukraine in exchange for a reopening of an investigation into Biden's son, he would support an impeachment inquiry.

"I don't think we have a choice under the Constitution. We must move forward with impeachment proceedings. I don't think we'll have much of a choice," Gonzalez said.

New subpoena threat

The new comments from Democratic lawmakers Monday comes as three House committees threatened to subpoena the State Department over "documents the State Department has been withholding in response to a letter the Chairmen sent two weeks ago relating to efforts by the President and his personal lawyer, Rudy Giuliani, to solicit the help of a foreign power to interfere in the 2020 Presidential Election." Three chairmen -- House intelligence Chairman Adam Schiff, Foreign Affairs Chairman Eliot Engel and Oversight Chairman Elijah Cummings -- authored a letter Monday to Secretary of State Mike Pompeo.

"Seeking to enlist a foreign actor to interfere with an American election undermines our sovereignty, democracy, and the Constitution, which the President is sworn to preserve, protect, and defend," the three chairmen wrote jointly. "Yet the President and his personal attorney now appear to be openly engaging in precisely this type of abuse of power involving the Ukrainian government ahead of the 2020 election."

Trump acknowledged on Sunday that he discussed the former vice president in a July call with Ukraine's president. CNN has previously reported that Trump pressed Ukrainian President Volodymyr Zelensky in the call to investigate Biden's son, Hunter, according to a person familiar with the situation. That call was also part of the whistleblower complaint submitted to the Intelligence Community Inspector General, which hasn't been shared with lawmakers despite calls from Democrats. There is no evidence of wrongdoing by either Joe or Hunter Biden.

This story has been updated with additional developments Monday.