The storm over Trump's self-admitted call for Ukraine's President to investigate his potential 2020 election foe Joe Biden may turn out to be his most perilous political scandal.

But it is also threatening to obliterate the House speaker's fragile political perch atop her caucus, a majority of which rejects her reluctance to open impeachment proceedings.

Pelosi, trading on her reputation as the master of her troops, has argued that the best way to deal with Trump is to oust him at the next election . She fears that a backlash from impeachment could electrify Trump's base and doom Democrats who helped her win back the House last fall. And since Republicans will almost certainly never help to produce the two-thirds majority needed to convict Trump in a Senate trial, the argument is that Democrats are walking into an impeachment trap.

The speaker has been trying to build a case to 2020 voters that Trump is guilty of obstruction and is unfit for office -- but has always stopped short of pulling the impeachment trigger.

"The President is almost self-impeaching because he is every day demonstrating more obstruction and disrespect for Congress' legitimate role to subpoena," Pelosi said in May. But the latest twists over Ukraine suggest that the politics of impeachment may be changing inside the House Democratic caucus and that the President's accusations that Biden is guilty of corruption -- for which there is no evidence -- may be consuming her maneuvering room.

Pelosi is also facing increasing criticism among commentators who believe her strategy enables ever bolder power grabs by the President. And 2020 Democratic presidential hopefuls are becoming ever more vocal in calling for impeachment.

Seven freshman Democrats who are also veterans of the military and defense and intelligence agencies penned an op-ed in The Washington Post warning that if Trump had threatened to withhold aid to Ukraine in order to force it to help him in the election it would be an impeachable offense. Trump had ordered a hold on nearly $400 million of military and security aid to Ukraine at least a week before the call in question, US officials familiar with the matter tell CNN.

"These new allegations are a threat to all we have sworn to protect. We must preserve the checks and balances envisioned by the Founders and restore the trust of the American people in our government," they wrote.

Pelosi is showing signs she's 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. But she left little doubt the developments around the whistleblower's complaint had dramatically escalated the standoff with Trump and a move toward impeachment proceedings was all but certain.

"We will have no choice," Pelosi said of ultimately initiating an impeachment inquiry.

During the flight, Pelosi read the op-ed from seven freshman Democrats and 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 dam could break

And Trump's brazen behavior suggests he is far from cowed by the current Democratic oversight offensive, which he stymied with executive privilege claims and court challenges. Last week's Capitol Hill appearance by his former campaign manager Cory Lewandowski, who put on a show of obstruction and outrage that delighted his boss, threatened to turn the whole process into a farce.

JUST WATCHED See the questions Lewandowski refused to answer Replay More Videos ... MUST WATCH See the questions Lewandowski refused to answer 02:44

Trump's approach to Kiev -- which he basically admitted on Sunday -- suggests he will stop at nothing to win reelection. Some Democrats already think the 2016 election was stolen. If they come to believe the 2020 vote could be compromised, Pelosi will come under extreme pressure to do something with the power that she won in the 2018 midterm election. And she may come to consider history's judgment of her strategy.

There is also a chance that if the controversy keeps escalating, Senate Democrats could warm to the idea of jamming Republicans in 2020 with the toughest vote of their careers -- to convict or acquit an unpopular President.

If at some point, the Ukraine controversy convinces leading Democrats that the political liabilities of not moving against Trump outweigh the liabilities of doing so, the dam could break. The same could be the case if Democrats come to believe that they can damage Trump more by voting on articles of impeachment than letting him off the hook.

Pelosi's warning over the weekend that if the administration did not hand over to Congress a whistleblower's complaint on the Ukraine affair Democrats would have to move to a "whole new stage of investigation" appears to hint at this new reality. She didn't use the word impeachment, but her words were left hanging hours after her ally, House Intelligence Committee Chairman Adam Schiff said on CNN impeachment could be the only remedy.

JUST WATCHED Schiff on impeachment: may have 'crossed the Rubicon' Replay More Videos ... MUST WATCH Schiff on impeachment: may have 'crossed the Rubicon' 03:01

CNN's Manu Raju reported on Monday that Democrats, including freshmen moderates from districts Trump won, are privately telling colleagues they could embrace impeachment if the storm intensifies.

The tipping point would be if there's evidence that Trump sought to withhold the aid to Ukraine in exchange for Kiev opening a probe into the Bidens, sources said.

Democratic Rep. Jim Himes of Connecticut told CNN's Poppy Harlow on Monday that it was clear that Trump had crossed a line.

"I mean, extorting a foreign leader for the purposes of getting that leader to do your political work, to try to find dirt on your opponent is extortion," he said. "Of course, it's an impeachable offense."

But Himes also explained the nuanced politics that have undergirded Pelosi's political thinking so far and her desire to follow public opinion -- not the impulses of her most radical members on the question of impeachment. He pointed out that the next election will come down to a few states and that Democrats should not doing anything that compromises their chances.

"It is not fear of Donald Trump. It is fear of a second Trump term," he said. "Now, I happen to believe that an impeachment inquiry would actually probably, at worst, be neutral in those states. But that's what's going on here."

Trump fights back

How the drama will unfold in the coming days is difficult to predict, because it remains unclear exactly what Trump said to President Volodymyr Zelensky of Ukraine.

But in a possible sign that his remarks could contain material that could be portrayed as incriminating, Trump went on a tear at the UN General Assembly on Monday. It was classic Trump, as he sought to whip up a narrative that will stir his political base and offer fodder for the conservative media. He gave no quarter, showed no sense of personal vulnerability and, in another familiar play, accused someone else of a charge to which the President himself has often seemed vulnerable.

"Joe Biden and his son are corrupt." Trump said. "If a Republican ever did what Joe Biden did, if a Republican ever said what Joe Biden said, they'd be getting the electric chair right now."

There is no evidence that Biden or his son Hunter did anything corrupt in Ukraine. Authorities in Kiev have said that Biden's son did not break the law by joining the board and taking money of a prominent energy firm.

The President also said on Monday that he did not threaten to withhold military aid that Ukraine needs to stand up to Russia to force Zelensky to investigate Biden.

It's possible that a transcript of his conversations with the Ukrainian President could leave Trump sufficient wiggle room for his explanations to be seen as credible and for the scandal to join the churning mix of previous controversies that have tarnished his presidency but never felled him. But the fact that the administration is trying to prevent the whistleblower's report on the affair from being handed to Congress is fueling suspicion about his behavior.

And in theory at least, there might not need to have been a specific quid pro quo for Trump to be judged to have abused the power of his office and the public trust. Any pressure on a vulnerable foreign leader to target a domestic opponent would represent severe overreach by the President, a fact one Trump adviser recognized by saying, "This is a serious problem for us" adding "he's admitted doing it."

Still, there is an almost endless list of incidents, comments and transgressions that many Washington observers thought represented the last straw for Trump. And he's ridden out them all.

So in the end, Pelosi may have to decide whether public opinion would sustain an impeachment drive against Trump -- or whether it could end up hurting Democrats.

In other words, she'd face a new dimension of the same dilemma she's faced all along. No one can say yet if her answer would be different.

This story has been updated to reflect additional reporting on the nearly $400 million in military and security aid to Ukraine that was held.