– A growing number of House Democrats are publicly calling for a formal inquiry into President Donald Trump's impeachment amid continued stonewalling from his administration, applying new pressure to Speaker Nancy Pelosi and other party leaders who have been determined to stick to a methodical course of investigation and litigation.

While a handful of lawmakers have agitated for impeachment for months, Tuesday's no-show of former White House counsel Donald McGahn at a House hearing and the uncertain prospects for public testimony from special counsel Robert Mueller have prompted a larger number of Democrats to speak out. On Monday and Tuesday, 25 House Democrats publicly called for an impeachment inquiry.

Leading the charge are members of the House Judiciary Committee who have been increasingly frustrated with the Trump administration's blanket refusal to cooperate with congressional requests for documents and testimony. Some confronted Pelosi at a private meeting of the House leadership Monday, seeking to convince her that an impeachment probe would be the most effective way to hold Trump to account, even if he is never formally impeached.

"This effort by the president and the White House to impede and undermine our ability to collect the evidence necessary to do our work is something that can't be tolerated," said Rep. David Cicilline, D-R.I. who sits on the Judiciary Committee and is a member of the party leadership team. Democrats, he said, need to "demonstrate that you cannot just trash the Constitution, undermine the rule of law, and expect the Congress of the United States to accept that."

Pelosi has so far carefully managed the pressure, refusing to rule out an impeachment of Trump while also emphasizing the need to pursue a prudent course to defend congressional prerogatives against presidential resistance.

McGahn's no-show has particularly rankled Democrats who have been exasperated by Trump's sweeping claims of immunity from congressional oversight. McGahn told lawmakers he would skip the hearing pursuant to a White House request rooted in a new Justice Department opinion barring testimony from close presidential advisers.

The panel held a 23-minute "empty chair" hearing Tuesday, during which Chairman Jerrold Nadler, D-N.Y., vowed to continue his probe and accused Trump of seeking to intimidate McGahn into not appearing, calling that "not remotely acceptable."

"We will hold this president accountable, one way or the other," he said, holding out the possibility of a contempt-of-Congress charge. "Our subpoenas are not optional."

Nadler on Tuesday issued new subpoenas to Annie Donaldson, McGahn's former chief of staff, and Hope Hicks, Trump's former White House communications director. Both, like McGahn, cooperated extensively with Mueller and were key sources for his report.

Doubly frustrating to many Democrats is that Mueller himself has yet to appear before Congress more than a month after he delivered his 448-page, redacted report.

Nadler wants Mueller to testify as soon as possible, but the committee and the special counsel's office have been locked in dispute over whether the hearing will be public or private, according to people familiar with the talks who spoke on the condition of anonymity.

The newly vocal Democrats said they are seeking only an impeachment "inquiry" — a formal investigation that may or may not lead to the actual drafting and passage of articles of impeachment to be tried by the Senate.

But for a broader group of lawmakers, that is a distinction without a difference. They fear that any rush into impeachment proceedings would betray campaign promises to focus on policy issues more directly affecting their constituents, a potentially perilous political move ahead of the 2020 elections.

Pelosi scheduled a Democratic caucus meeting for Wednesday morning, billing it as an opportunity for members to receive updates on oversight and investigations. But many lawmakers said Tuesday that they expect it will become a robust discussion of whether to pursue an impeachment inquiry.