A slew of key swing-district Democrats late Monday threw their support en masse behind opening a formal impeachment inquiry, as The New York Times, The Washington Post and The Wall Street Journal reported in the evening that President Trump personally ordered acting White House chief of staff Mick Mulvaney to freeze nearly $400 million in military aid to Ukraine just days before he pressed the new Ukrainian president to investigate Joe Biden and his son Hunter.

The rapid-fire declarations by the influential Democrats, after seemingly months of teetering on the brink, came just a day after House Speaker Nancy Pelosi strongly suggested she was now warming to the idea of impeaching the president. Speaking to Fox News, a House Democratic leadership aide confirmed that the Democratic caucus will meet Tuesday afternoon to discuss the Ukraine matter and how to proceed.

"The horse is out of the barn," said former Hillary Clinton pollster and strategist Geoff Garin. "Saddle up."

In remarks to reporters at the United Nations on Monday, Trump denied linking the aid money to Ukraine's investigative actions. “No, I didn’t — I didn’t do it,” Trump said. He also called the Bidens' actions in Ukraine a "disgrace," and added: “It’s very important to talk about corruption. ... Why would you give money to a country that you think is corrupt?”

Joe Biden has acknowledged that, when he was vice president, he successfully pressured Ukraine to fire its top prosecutor, Viktor Shokin, while Shokin was investigating the natural gas firm Burisma Holdings — where Hunter Biden was on the board.

Trump has long said he wants European countries to pay more for their own defense, and an administration official told the Journal that Trump's actions on the call with Ukrainian President Volodymyr Zelensky in July "reflected the president’s concerns about how the U.S. is spending aid money and whether U.S. allies are adequately contributing."

Another official told the paper that the Trump administration internally maintained at the time that Ukraine's corruption problems were a consideration in the aid decision. Trump's decision to freeze Ukraine funding reportedly came more than a week before his call with Zelensky.

"The horse is out of the barn. Saddle up." — Geoff Garin

Fox News has not independently confirmed the reports, which rocked Capitol Hill even as Democrats insisted they were keeping the developments in perspective. While Democrats are set to meet Tuesday to discuss Ukraine and possible impeachment proceedings, the top House aide told Fox News that trade – not Ukraine – would headline Democrats' agenda at a separate planned caucus meeting Wednesday.

"The dominant focus of the caucus on Wednesday is trade," the aide told Fox News. "A second caucus has been added to ensure adequate time for member discussion on the whistleblower matter and a number of other pressing matters on Tuesday afternoon."

Something of an impeachment inquiry dam broke late Monday, when seven centrist Democratic freshmen lawmakers who served in the military and national security announced in an op-ed that if Trump pressured Zelensky to investigate the Bidens for political benefit, it's impeachable. The lawmakers – Reps. Gil Cisneros of California, Jason Crow of Colorado, Chrissy Houlahan of Pennsylvania, Elaine Luria of Virginia, Mikie Sherrill of New Jersey, Elissa Slotkin of Michigan and Abigail Spanberger of Virginia – come largely from swing districts where Trump is popular but voters split.

The Democrats wrote in The Washington Post they "do not arrive at this conclusion lightly."

“These allegations are stunning, both in the national security threat they pose and the potential corruption they represent," the lawmakers said. “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. And that is what we intend to do.”

Later in the night, Hawaii Democratic Sen. Brian Schatz and Michigan Democratic Rep. Debbie Dingell also endorsed an impeachment inquiry. Such an inquiry would not be an impeachment vote, but rather an investigative process that could quickly lead to actual impeachment.

"On behalf of the people who elected us, we must formalize and accelerate the impeachment process so that Congress, by exercising its responsibility under Article 1 of the Constitution, can provide some measure of accountability," Schatz said in a statement.

The flurry of moves brings to 154 the number of House Democrats who have signaled strong support for possible impeachment proceedings. (235 Democrats and 198 Republicans are in the House, with one independent -- and a majority would be required to successfully impeach the president. An unlikely two-thirds vote in the GOP-controlled Senate would be needed to convict and remove the president.)

Democrats have presented conflicting views, in court and in public, as to whether impeachment proceedings are already in progress. The renewed push on Monday could galvanize remaining Democrats to openly call their efforts an impeachment probe.

FLASHBACK: IN APRIL, TRUMP SAYS HILLARY-UKRAINE COLLUSION ALLEGATIONS 'BIG,' WILL BE INVESTIGATED

Meanwhile, a report in Politico late Monday suggested that Democrats may vote on a resolution to condemn Trump for allegedly pressuring Ukraine's leadership to investigate possible corruption by Joe Biden's son Hunter relating to his business dealings in the country.

And The Washington Post reported in the evening that Pelosi was sounding out members of the Democratic caucus to see if the time has come to impeach the president.

However, earlier Monday, a person familiar with the situation told Fox News that the whistleblower in question did not have "firsthand knowledge" of Trump's conversation with Zelensky.

The source said that it was made clear in the complaint itself that the whistleblower did not have direct knowledge of the July phone call.

Fox News has learned that typically, multiple U.S. officials would be on such calls with the president, but this would indicate the whistleblower was not one of those people. It's unclear if the individual read a transcript of the call, heard about it in conversation, or learned of it another way.

SOURCE TO FOX NEWS: WHISTLEBLOWER HAD NO FIRST-HAND KNOWLEDGE OF TRUMP CALL

Nevertheless, Democrats have pressed forward on the issue, even as it risked backfiring by exposing potential wrongdoing by the former vice president.

"The Senate Republican 'see no evil, hear no evil' attitude is unacceptable and must change,” Senate Minority Leader Chuck Schumer said on Senate floor Monday. "This again is an issue of solemn obligation. There is no wiggle room here, none."

The New York Democrat earlier in the day wrote a letter to Senate Majority Leader Mitch McConnell asking him to issue a subpoena and hold hearings over the complaint.

In a floor speech Monday afternoon, the Kentucky Republican rebuffed those calls from Democrats, saying the Senate Intelligence Committee has been following an established process and was working to hear from the intelligence community's inspector general this week.

"It is regrettable that House Intelligence Committee Chairman [Adam] Schiff and Sen. Schumer have chosen to politicize the issue," McConnell said, calling for looking into the matter in an "appropriate, deliberate bipartisan manner."

At least one Republican lawmaker has also called for the release of the transcript of Trump's call.

"At this stage, given the seriousness of the allegations, it’s very important that the transcript and potentially as well the whistleblower come forward," said Utah Republican Sen. Mitt Romney, an occasional Trump critic and onetime presidential rival.

UKRAINE CONTROVERSY BOILS OVER ON SENATE FLOOR

Meanwhile, top Republicans including California Rep. Devin Nunes predicted in interviews with Fox News that the gambit would backfire for Democrats.

And in an exclusive interview with Fox News' "Hannity" on Monday, Vice President Mike Pence said Democrats' accusations were about to boomerang.

"The ironic thing is is that everything that our critics in the media are leveling at the president from this phone call, and leveling at our administration, everything that Democrats on Capitol Hill are running off and describing -- Vice President Joe Biden bragged about -- which was a quid-pro-quo -- withholding American aid in exchange for a specific action," Pence claimed.

Late Sunday evening, Trump sounded a similar note, tweeting 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."

Indeed, the whistleblower's allegation could prompt scrutiny of the Obama administration's Ukraine policy. The former vice president 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. The prosecutor, at the time, was investigating possible corruption involving a natural gas firm with close ties to Hunter Biden.

"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.

At the same time, 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.

The whistleblower drama kicked into gear after The Wall Street Journal reported Friday that Trump repeatedly had asked Zelensky to investigate Hunter Biden.

Acting Director of National Intelligence Joseph Maguire is expected to testify before the House Intelligence Committee at an open hearing on Thursday. Democrats have sought the full whistleblower complaint and testimony from the whistleblower.

But Maguire has refused to share that information, citing presidential privilege.

"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, D-Calif., warned Sunday. "Thank you for your patriotism."

Pelosi went on: "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."

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."

The top Democrats' rhetoric came soon after Rep. Alexandria 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."

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Pelosi and Ocasio-Cortez, D-N.Y., 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.

Fox News' Alex Pappas, Jake Gibson, and The Associated Press contributed to this report.