(CNN) The Ukraine scandal raging around Donald Trump is forcing Democrats to confront a fateful choice on impeachment that will not just shape the 2020 election but will echo down the ages.

The facts of whether the President pressured Ukraine to investigate his potential Democratic general election opponent Joe Biden while a US military aid package was on the table are still obscured.

Trump supporters say there is so far no evidence that he offered a quid pro quo to the Ukrainians and note that an intelligence community whistleblower who raised the alarm was operating with a second-hand knowledge of Trump's conversations.

But if Trump used his power to try to coerce a foreign leader into influencing US elections, it could precipitate the worst political crisis of a presidency that has been mired in notoriety from its first hours.

It would amount to a situation in which Trump's team, which according to the Mueller report expected to benefit from Russian election meddling in 2016, is now using the power of the presidency to incite collusion ahead of the 2020 election.

That possibility seemed to unlock a shift Sunday in the Democratic position on impeachment. House Intelligence Committee chairman Adam Schiff said on CNN that Trump may have "crossed the Rubicon." And House Speaker Nancy Pelosi -- who has been loath to contemplate an impeachment drama -- warned that events might necessitate a "new stage of investigation."

Rep. Gerry Connolly, a Democrat on the House Oversight and Foreign Affairs panels, told CNN on Monday morning, "I think we're reaching a tipping point both within our base and within our caucus."

Republican senators who have strongly supported Trump were largely silent but Republican Sen. Mitt Romney of Utah tweeted Sunday "it would be troubling in the extreme" if Trump pressured Ukraine to investigate Biden and that it is "critical for the facts to come out."

There is no evidence of wrongdoing by Biden or his son Hunter. The then-Ukrainian prosecutor general Yury Lutsenko said in May that Burisma Holdings, a major energy company, did not violate Ukrainian law by having Hunter Biden on its board and paying him.

Trump's claims that Biden pushed for the firing of a Ukrainian prosecutor because he investigated a business for which his son served as a board member have previously been found to be false. The prosecutor was seen as corrupt by multiple governments and international institutions, not just the former vice president.

Why pressure by Trump on Ukrainians would matter

Presidents are expected to act in the interests of all Americans and not to use their vast discretion in foreign policy to pursue political vendettas or subvert US democracy. The Founders saw the presidency as a public trust, meaning that its incumbents should not put their personal interests over the national interest. The Ukraine story is so significant because it may have the potential to fall into such grave constitutional territory and could represent an abuse of presidential power.

Trump and his team seemed at odds Sunday over whether to publish the transcript of his conversations with Ukrainian President Volodymyr Zelensky.

And they went on offense in typical fashion, bending facts and spinning conspiracy theories, obfuscation and hypocrisy.

Trump insisted that he said "absolutely nothing wrong" in the call with the Ukrainian president. "It was perfect," he said. Trump often gives the impression that he believes he is not constrained by norms on the limits of power observed by past presidents. In July for instance, he said, falsely, that Article 2 of the Constitution "allows me to do whatever I want as President."

Past scrapes like the 2016 Russian election meddling scandal -- and multiple controversies ranging from his insulting behavior toward the late Sen. John McCain to his payments to women who claimed they had affairs with him -- have failed to bring him down. His emergence from each may have taught him a lesson.

Trump says he spoke to Ukrainian President about Biden

Trump on Sunday appeared to add new context to the Ukraine story when he said that he did indeed discuss Biden with Ukraine's president at a time when Kiev was awaiting a $250 million military aid package from the United States. The call with Zelensky took place on July 25. Congress passed the bill in August and the White House lifted a hold on the money in September.

"The conversation I had was largely congratulatory, with all of the corruption taking place and largely the fact that we don't want our people like Vice President Biden and his son creating to the corruption already in the Ukraine," Trump said.

CNN has reported that Trump urged Zelensky to investigate Biden's son in a call on July 25, but did not discuss a pending aid package at the time, indicating there may not have been an explicit quid pro quo outlined in the conversation.

The latest developments highlighted Pelosi's reluctance to trigger impeachment proceedings against Trump amid fears of a political backlash. But Schiff, a Pelosi ally, suggested things may be about to change.

"This would be an extraordinary remedy, a remedy of last resort and not first resort," Schiff said Sunday on CNN's "State of the Union."

"But if the President is essentially withholding military aid at the same time he is trying to browbeat a foreign leader into doing something illicit, providing dirt on his opponent during a presidential campaign, then that may be the only remedy that is co-equal to the evil that that conduct represents."

Pelosi and Schiff were in close coordination throughout the weekend talking about the Ukraine whistleblower story and coordinating strategy, a leadership aide confirmed to CNN.

The way that events could force the hands of Democratic leaders became even clearer later on Sunday.

Pelosi hinted at a change of strategy in a message to Democratic colleagues, over half of whom had already backed the idea of impeachment.

"If the Administration persists in blocking this whistleblower from disclosing to Congress a serious possible breach of constitutional duties by the President, they will be entering a grave new chapter of lawlessness which will take us into a whole new stage of investigation," Pelosi wrote on Sunday.

Democratic reluctance on impeachment

Pelosi has been reluctant to embrace impeachment since Senate Republicans are unlikely to vote to convict the President. There is also no desire to set Trump's political base alight as Democrats try to keep their House majority in 2020.

If Trump's behavior is exposed as corrupt, Democrats may be forced into impeachment hearings -- whatever the long-term political cost.

To do nothing would be to accept that a President can abuse his power by seeking foreign interference in American democracy. Trump would feel validated and emboldened.

The balance between Congress and the Presidency will have been fundamentally altered and there will be few checks and balances left capable of constraining Trump and future presidents.

Inaction might also be politically unsustainable since Democrats might see their own leaders as willing to use the power of a House majority to defend their own presidential front-runner.

Democrats, both freshmen from Trump-won districts and moderates who have resisted calls for impeachment, are telling their colleagues privately they are prepared to announce their support for impeachment proceedings if the controversy continues to grow -- namely if there's evidence that Trump sought to withhold military aid to Ukraine in exchange for investigating the Bidens, according to Democratic sources involved in the conversations on Monday morning.

That could change the calculus for Pelosi, who has in part resisted because she has sought to protect vulnerable Democrats in GOP-leaning districts who could face voter backlash over impeachment. But if those members come out in support of impeachment proceedings, Pelosi is bound to shift her positioning as well, according to Democrats close to the speaker.

Trump allies hit back

Seeking to fog such questions, Trump's lieutenants went on the offensive on Sunday talk shows using a familiar playbook.

Secretary of State Mike Pompeo revived debunked questions about Biden's conduct.

"I do think if Vice President Biden behaved inappropriately, if he was protecting his son and intervened with the Ukrainian leadership in a way that was corrupt, I do think we need to get to the bottom of that," Pompeo said on ABC's "This Week."

"America cannot have our elections interfered with, and if that's what took place there, if there was that kind of activity engaged in by Vice President Biden, we need to know."

For the administration to make an argument about electoral interference seems somewhat rich, given that Trump has long rejected evidence that Russia intervened in 2016 to help him.

On "State of the Union," Treasury Secretary Steven Mnuchin said there was no reason to believe Trump pressured Ukraine, though admitted he wasn't on the call in question.

And he argued that Biden's son should not have been allowed to do business in Ukraine while his father was vice president. Asked by Jake Tapper Sunday about the glaring inconsistency in this statement since Trump's children continue to work globally on a business from which the President has not fully divested, Mnuchin dodged.

"I don't really want to go into more of these details," Mnuchin said.

Both Mnuchin and Pompeo opposed releasing transcripts of Trump's calls, arguing that a President has a right to confidentiality in conversations with foreign leaders.

The administration's efforts to stop acting Director of National Intelligence Joseph Maguire from releasing the whistleblower report to Congress, and its opposition to releasing a transcript, are only fueling speculation the White House has something to hide. If there was wrongdoing such conduct could equal obstruction of justice, historically an impeachable offense.

Trump, however, said that he hoped they would release the transcript.

Biden, meanwhile, spent the weekend defending himself and trying to turn the scandal to his own advantage in a tight primary race.

"Trump is doing this because he knows I'll beat him like a drum and he is using the abuse of power and every element of the presidency to try to do something to smear me," Biden said.

This story has been updated with additional developments Monday.