When Mueller noted that Congress, and not he, had the power to hold Trump to account and pointedly refused to exonerate the President of criminal conduct, momentum swiftly picked up toward a fateful political moment. Democratic presidential hopefuls demanded impeachment and liberal lawmakers grew perceptibly more restless as a media frenzy ensued.

Then came Pelosi with the cold water.

"You don't bring an impeachment unless you have all the facts," the unruffled speaker said in California several hours after the special counsel spoke on his last day at work in Washington.

There may come a moment when Pelosi senses it's time for Democrats to move forward on the divisive business of seeking to oust an elected President. If not, her position -- which appears to be running counter to the direction of her party's evolution on impeachment -- could become unsustainable.

While Pelosi still has the support of her most powerful committee chairs, the building political pressure means that may not always be the case.

And the swift reactions of 2020 presidential candidates who have all spent weeks meeting party activists may be an early sign of shifting Democratic opinion.

In Washington, the most successful leaders apply power through maximizing their leverage, mastering political timing and assessing the cost/benefit ratio of any move ahead of time. And Pelosi's public comments show she doesn't believe such tests have yet been met.

The critical point could be reached when a true constitutional crisis erupts -- should Trump for instance refuse an eventual court order to hand over documents to Congress.

If the President's blanket policy of non-cooperation and aggressive assertions of executive privilege begins to truly threaten the bedrock principles of the separation of powers, things could escalate.

Such moves might shift polls that currently suggest most Americans do not want the national agony of an impeachment drama for the third time in 50 years.

But the pivot requires a shift in wider perceptions of the Trump presidency and will only come when Democratic leaders conclude that Republicans will pay more of a political price for shielding Trump than they will pay for pursing impeachment.

Public sentiment can change throughout an impeachment process -- it shifted perceptibly against President Richard Nixon in 1974 -- but Pelosi is arguing America is not there yet.

"We won't be swayed by a few people who think one way or another who are running for president as much as I respect all of them and they have the freedom to be for impeachment," Pelosi said.

"We have the responsibility to get a result for the American people and that's where we're going."

Democrats fret about constitutional duties

JUST WATCHED Nadler: Mueller clearly demonstrated that Trump is lying Replay More Videos ... MUST WATCH Nadler: Mueller clearly demonstrated that Trump is lying 02:09

The speaker's refusal to shift her ground is already focusing attention on her strategy. Democratic presidential candidates want a more robust offensive against Trump.

Democratic Sen. Elizabeth Warren of Massachusetts warned that if Congress didn't act "we have fundamentally violated our duties under the Constitution, and changed how this country operates."

Her 2020 rival, Sen. Kamala Harris of California said: "Bob Mueller was essentially referring impeachment to the United States Congress."

Pelosi managed to quell calls for impeachment during a meeting with her caucus last week in Washington, pointing out several key court victories in the fight to investigate the White House.

It looks like she may have to do the same again when Congress is back from recess next week.

Democratic House members are "growing more restless," one of their number told CNN's Jim Acosta on Wednesday.

Mueller's implication that he may have indicted Trump but for Justice Department rules prohibiting charges against sitting presidents seemed to strengthen the case for impeachment.

He also rebuked the President's claims that his probe was a "witch hunt" by praising the integrity of his team, and his comments clearly contradicted Attorney General William Barr's controversial summaries of his report , which helped to set a favorable political narrative for the President.

Mueller's appearance sharpened the knife edge on which Pelosi is operating. She is caught between possibly irreconcilable responsibilities -- keeping the House and winning back the White House for Democrats in 2020 which she sees as endangered by an unsuccessful impeachment push -- and her institutional duty to confront apparent evidence of presidential abuses of power.

There is queasiness among Democratic leaders who fear that an unsuccessful bid to oust Trump from power will only strengthen the President in the run-up to the 2020 election and will validate his norm busting leadership style.

White House spin

The White House is vigorously spinning Mueller's appearance as nothing new -- and sticking to its line that there was no collusion or conspiracy with Russia by Trump's campaign and no obstruction of justice by the President.

"We consider this very much to be case closed," said White House press secretary Sarah Sanders.

Her comments reflected the fact that Senate Republicans are no more likely to vote to cut the President loose in a Senate impeachment trial than they were before Mueller spoke.

And Trump seems to be itching for an impeachment battle as a base stoking device. A source familiar with the President's thinking explained his attitude on impeachment to CNN.

"Let's do it. This fight will end up on our side. American people will see this as a scam. It is," the source said.

Pelosi's position comes with risks. A trickle of Democratic members of Congress came out for impeachment following Mueller's appearance on Wednesday. So far, according to a CNN tally, 38 Democratic House members have declared for impeachment, along with dissident Republican Rep. Justin Amash.

Some observers are warning that the Speaker's concern for political factors could be compromising her institutional role.

"Mueller was sending a message to Speaker Pelosi and the House that they are constitutionally obligated to act," said Corey Brettschneider, a professor of political science at Brown University.

"If the House fails to begin impeachment proceedings that would be allowing the President to get away with a crime and essentially would place him above the law, the inverse of the idea that we are subject to the rule of law," Brettschneider, author of the book "The Oath and the Office: A Guide to the Constitution for Future Presidents," said.