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(CNN) —  

Not even a sudden, silence-shattering prod from Robert Mueller could force Nancy Pelosi’s hand on impeachment.

But the special counsel’s dramatic public statement Wednesday energized growing enthusiasm for a more aggressive assault on President Donald Trump among many Democrats even as the House speaker tried to hold them back.

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

02:09 - Source: CNN
Nadler: Mueller clearly demonstrated that Trump is lying

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 re