Just hours after Rep. Alexandria Ocasio-Cortez, D-N.Y., said her party's failure so far to impeach President Trump amounted to a major "national scandal," House Speaker Nancy Pelosi appeared to relent on Sunday -- and warn that a "whole new stage of investigation" could be approaching.

Pelosi, who has long resisted calls to impeach Trump because the move would imperil the electability of moderate Democrats in the House, outlined her new stance in a statement Sunday. Pelosi specifically said that a whistleblower who reportedly alleged that Trump acted improperly during a July 25 phone call with Ukrainian President Volodymyr Zelenskiy must be allowed to speak to Congress, and the full whistleblower complaint must be provided.

Her aggressive push came as top Republicans, including California Rep. Devin Nunes, predicted that the gambit would backfire for Democrats. Later in the evening, Trump sounded a similar note, saying the "real story" was that "Sleepy Joe Biden" had "forced a tough prosecutor out from investigating his son's company by the threat of not giving big dollars to Ukraine."

The whistleblower drama escalated after The Wall Street Journal reported Friday that Trump repeatedly had asked Zelenskiy to investigate Hunter Biden, the former vice president's son who had a key role in a natural gas firm that a Ukrainian prosecutor had been investigating as part of a corruption probe. At a conference two years after he left office, Joe Biden openly bragged about successfully pressuring Ukraine to fire that prosecutor when he was vice president.

CNN later reported the whistleblower had no direct knowledge of the call, and a top Ukrainian official on Saturday defended Trump's actions. Unverified reports circulated on left-leaning media outlets claiming that Trump could have promised something improper in exchange for Ukraine's compliance, although the Journal reported there was no "quid-pro-quo" involved.

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But if Democrats' demands were not met, Pelosi said, the Trump administration's "grave new chapter of lawlessness" would necessitate drastic action.

"We must be sure that the President and his Administration are always conducting our national security and foreign policy in the best interest of the American people, not the President’s personal or political interest," Pelosi said.

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Speaking to CNN's "State of the Union," House Intelligence Committee Chairman Adam Schiff, D-Calif., also said impeachment was on the table. Schiff previously has claimed to have hard evidence that the Trump team improperly colluded with Russia.

“Why doesn’t the president just say, ‘Release the whistleblower complaint.’ Clearly he’s afraid for the public to see,” Schiff said. “This would be the most profound violation of the presidential oath of office, certainly during this presidency, which says a lot, but perhaps during any presidency. There is no privilege that covers corruption. There is no privilege to engage in underhanded discussions.”

“We may have crossed the Rubicon here,” he added, referring to the river that Julius Caesar fatefully traversed in 49 BC. “I have been very reluctant to go down the path of impeachment. ... but that may be the only remedy that is co-equal to the evil that that conduct represents.”

The top Democrats' rhetoric came soon after Ocasio-Cortez on Saturday night kickstarted a dormant, but long-simmering and occasionally explosive feud with top House Democrats in the form of a fiery tweet: "At this point, the bigger national scandal isn’t the president’s lawbreaking behavior - it is the Democratic Party’s refusal to impeach him for it."

Pelosi and Ocasio-Cortez appeared to reach a truce of sorts after a closed-door meeting in July, in which both sought to ease infighting that some Democrats viewed as counterproductive. That meeting came shortly before the departure of Ocasio-Cortez’s chief of staff, Saikat Chakrabarti.

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Trump acknowledged Sunday that he had communicated with Zelensky about Biden, and that the conversation concerned "the corruption taking place and largely the fact that we don't want our people like Vice President Biden and his son [contributing] to the corruption already in the Ukraine."

The president and top officials maintained Sunday that nothing inappropriate occurred on the call.

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Rudy Giuliani, the president's personal attorney, has long said publicly he wanted Ukraine to look into the Bidens' dealings in Ukraine, and admitted this week that he's asked Ukraine to do so in the past -- but denied any impropriety or illegal conduct again on Sunday.

"This violation is about our national security," Pelosi said. "The Inspector General determined that the matter is 'urgent' and therefore we face an emergency that must be addressed immediately."

Inspector General Michael Atkinson of the office of the Director of National Intelligence (DNI) wrote in a Sep. 9 letter to the House Intelligence Committee that the whistleblower complaint "appeared credible" and related to an "urgent" matter. But, the DNI general counsel said days later that, after consulting with the DOJ, the matter did not meet the legal definition of an "urgent concern," and was not subject to mandatory disclosure to Congress.

Trump's conversation came as the White House was holding up $250 million in military aid for Ukraine, money that the White House later released. The president has said he wanted European countries to pay more for their own defense, and denied delaying any military aid funding.

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"Furthermore, because the complaint involves confidential and potentially privileged communications by persons outside the Intelligence Community, the DNI lacks unilateral authority to transmit such materials to the intelligence committees," Jason Klitenic, the DNI general counsel, wrote.

Acting Director of National Intelligence Joseph Maguire is expected to testify before the House Intelligence Committee at an open hearing on Thursday.

In her statement, Pelosi said Democrats would be watching closely.

"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 said. "Thank you for your patriotism."

Even some Republicans cautioned that the situation could be serious. Utah GOP Sen. Mitt Romney, a onetime Trump presidential rival, tweeted that "If the President asked or pressured Ukraine's president to investigate his political rival," that "would be troubling in the extreme."

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At the same time, California Rep. Devin Nunes predicted on Fox News' "Sunday Morning Futures" that Biden's campaign is likely coming to an end -- all because of the newly resurfaced reports about his possible misconduct in Ukraine that "first originated back when Hillary Clinton was trying to make sure Biden didn’t get in the race."

The top Republican on the House Intelligence Committee made the claim as The Des Moines Register/CNN/Mediacom poll showed Sen. Elizabeth Warren surging ahead of Biden as the first choice of 22 percent of the voters surveyed, while Biden was the first choice of 20 percent of the voters. Biden held a 9-point lead over Warren in the poll as recently as June.

Indeed, the whistleblower's allegation could prompt scrutiny of the Obama administration's Ukraine policy. Joe Biden has explained on camera that in March 2016, he privately threatened then-President Petro Poroshenko that the U.S. would withhold $1 billion in loan guarantees from Ukraine if its top prosecutor was not fired.

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"I said, 'You're not getting the billion,'" Biden recounted telling Poroshenko at a Council on Foreign Relations event. "I’m going to be leaving here in, I think it was about six hours. I looked at them and said: 'I'm leaving in six hours. If the prosecutor is not fired, you’re not getting the money.'"

"Well, son of a b---h, he got fired," Biden continued, after assuring Poroshenko that Obama knew about the arrangement. "And, they put in place someone who was solid at the time."

It remained unclear if this was directly tied to the prosecutor's probe into the company linked to Hunter Biden, as other countries reportedly wanted the prosecutor out as well.

And earlier this year, The Hill reported that the U.S. Embassy in Kiev, under the Obama administration, took the unusual step of pressuring prosecutors there to drop a probe into a group closely linked to liberal megadonor George Soros.

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Then, in April, Ukrainian law enforcement officials said they had a slew of evidence of collusion and wrongdoing by Democrats, and that they'd been trying to share this information with U.S. officials in the Justice Department.

A 2017 investigation by Politico found that Ukrainian officials not only publicly sought to undermine Trump by questioning his fitness for office, but also worked behind the scenes to secure a Clinton victory. Trump told Fox News that the allegations of possible Clinton-Ukraine collusion were "big" and vowed the DOJ would review them.

"The longer we talk about what the Bidens did in Ukraine, the better."
— Former Trump campaign adviser Barry Bennett

"The longer we talk about what the Bidens did in Ukraine, the better," said Barry Bennett, a former Trump campaign adviser, who dismissed those who claimed Trump will pay a political price for the latest controversy.

Meanwhile, Biden on Saturday denied he has ever spoken to Hunter about his business in Ukraine and called Trump's actions an "overwhelming abuse of power."

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

Trump, on Sunday, pointed out that Biden's claim was seemingly inaccurate. Hunter Biden told the New Yorker previously that he and his father had spoken "just once" about it.

"And now, he made a lie when he said he never spoke to his son," Trump said. "Of course you spoke to your son!"

Trump added: "No quid pro quo, there was nothing. It was a perfect conversation. ... The conversation I had was largely congratulatory, with largely corruption, 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 the corruption already in the Ukraine, and Ukraine has got a lot of problems. The new president is saying that he's going to be able to rid the country of corruption and I said that would be a great thing, we had a great conversation."

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Trump went on to say the latest allegations are "just as ridiculous as the others," branding it "the Ukraine Witch Hunt" — a nod to former Special Counsel Robert Mueller's Russia probe.

"Will fail again!" Trump tweeted.

Fox News' Ronn Blitzer, Fox Business Network's Maria Bartiromo and The Associated Press contributed to this report.