WASHINGTON — Speaker Nancy Pelosi portrayed herself Thursday as the protector of the Constitution, Congress and the country as House Democrats braced for war with President Donald Trump over his refusal to give them full access to special counsel Robert Mueller's report, related documents and witnesses.

"This is very methodical, it's very Constitution-based, it's very law-based, it's very factually based," Pelosi said about House plans to hold Attorney General William Barr in contempt for withholding documents. "It's not about pressure. It's about patriotism."

Trump and his Republican allies say Democrats are simply dressing up a partisan witch hunt in the haberdashery of constitutional principle. They express confidence that recent polls showing a lack of support for impeachment, particularly among independents, is evidence that the public agrees with them and that Democrats will only hurt themselves — and help the president — if they continue on their current course.

"If we’re already seeing that before any of the investigations begin, then moving toward impeachment will more than likely result in a backlash for Democrats," said one source close to the White House who asked to remain anonymous because he was not authorized to speak on behalf of Trump.

While the courts are likely to decide the scope of what Democrats can get their hands on, the fight over the terms of the public debate — partisan or constitutional — figures to have a significant impact on the political outcome, especially during a period in which the Trump's assertion of executive privilege limits the House's ability to produce any new evidence.

In any period in which one party controls the House and the other controls the White House, the impeachment process is inherently both a matter of solemn constitutional duty and partisan politics.

All of that helps explain why it's Pelosi's defense of another institution — the Democratic caucus — that is at the core of her approach to the investigations and possible impeachment of Trump. Though the cable talk shows and digital press have been full of speculation about what Pelosi wants — or believes — about impeachment, people who know her well say that she is driven in large measure by keeping solidarity in her ranks.

"She’s moving at a pace that all the spectrum of her caucus can tolerate right now," said former Maryland Rep. Donna Edwards. "She is very protective of the institution and the prerogatives of the institution, and you can see that, that she wants to insulate this from the politics and the electoral politics, and that is in keeping with her protection of the unity of the caucus."

In other words, when Trump's liberal critics put the impeachment cart before the process horse, moderate Democrats are quick to jump out. But when the question is framed as one of pursuing legitimate oversight of the executive branch, following investigations where they lead and maintaining the Constitution's balance of power, it is much easier for her to keep her troops in line.

In that way, Trump's actions have helped Pelosi start to resolve the conflicts in her caucus.

"There's a deep concern, particularly among institutionalists, about the balance of power," said a senior aide to one moderate Democrat who noted that the administration's refusal to comply with subpoenas has angered some lawmakers who had been reluctant to escalate the fight.

That is, the pace is speeding up even for most Democrats who have been reluctant to go down a path that could lead to impeachment.

Pelosi has said she believes Trump is "goading" Democrats into impeaching him, and many Republicans and Democrats in Washington believe that a House impeachment followed by a Senate acquittal would be a political gift to Trump and House Republicans.

There's even some concern among House Democrats that the very act of impeaching Trump would hand over power by giving the savvy Senate majority leader, Mitch McConnell, R-Ky., control of the timetable and process for trying the case with the 2020 election approaching.

And yet Trump's blanket defiance of Congress on the Mueller report and a range of other issues, from declining to provide his tax returns to declaring a national emergency so he could shift funds to build a border wall, has put Democrats in the position of acquiescing or escalating.

"The president and the attorney general have left the Congress, and the House in particular, not many choices," Edwards said.

For the moment, Democrats may have been handed some ammunition by an unlikely source as they try to make their case that Trump is tampering with the Constitution's checks and balances. It was reported Wednesday that the Senate Intelligence Committee, chaired by Sen. Richard Burr, Republican of North Carolina, issued a subpoena to Donald Trump Jr. after the panel was unable to secure a second round of testimony from him in its Russia probe.

While Trump Jr. is not an administration official — and the subpoena was actually sent in mid-April —the off-pitch sound from the GOP's previously harmonious message was discordant enough to trigger a response from Burr's fellow Republicans.

"The Mueller report cleared @DonaldJTrumpJr and he's already spent 27 hours testifying before Congress," GOP Sen. Thom Tillis, Burr's home-state colleague, wrote in a tweet. "Dems have made it clear this is all about politics. It's time to move on & start focusing on issues that matter to Americans."

I agree with Leader McConnell: this case is closed. The Mueller Report cleared @DonaldJTrumpJr and he’s already spent 27 hours testifying before Congress. Dems have made it clear this is all about politics. It’s time to move on & start focusing on issues that matter to Americans. https://t.co/11THs9LE0j — Senator Thom Tillis (@SenThomTillis) May 9, 2019

To win, Pelosi has to convince Americans that the fight is more about checks and balances than partisan politics — and that they should side with House Democrats over the Republican in the White House.

A divided caucus undermines that message. A united one helps to sell it.