Democrats in the U.S. House of Representatives are moving to formally start impeachment proceedings against President Donald Trump, in the wake of the president's confirmation that he withheld aid from Ukraine while at the same time urging the country's leader to investigate the son of former Vice President Joe Biden, who is running to be Trump's Democratic opponent in the 2020 presidential election.

Those developments, reportedly the subject of a whistleblower complaint made to the intelligence community's inspector general, have pushed House Speaker Nancy Pelosi, D-California, to publicly back the effort after she repeatedly declined to do so for several months.

Pelosi, in a televised statement delivered at the Capitol on Tuesday, said that "the actions taken to date by the president have seriously violated the Constitution."

The speaker specifically cited Trump administration officials' refusal to provide the whistleblower complaint despite it being labeled "urgent" by the inspector general for the intelligence community.

And she said that the House committees will continue with their investigations into the president—but now under the scope of an impeachment inquiry.

"The president must be held accountable," Pelosi said. "No one is above the law."

However, she offered few details about what exactly her announcement entails, including a timeline for when the House may reach an ultimate decision and vote on articles of impeachment.

Legal experts say these new claims—that the president sought to damage a political rival through the official capacity of his office—may prove to be more solid than the initial allegations of collusion investigated by former special counsel Robert Mueller III.

"It seems quite clear that request was backed up by an implicit or explicit threat or promise that aid would be forthcoming or withheld depending on what they did," Frank Bowman, a law professor at the University of Missouri, said of Trump's contacts with Ukraine.

And he said that, in the context of whether to start impeachment proceedings, it doesn't necessarily matter whether Trump's conduct was criminal or not. The Constitution states that a president can be impeached for "treason, bribery, or other high crimes," offering lawmakers broad discretion in what conduct fits that spectrum.

"The question is whether or not it is a high crime or misdemeanor," said Bowman, author of the book "High Crimes and Misdemeanors." "And I think it is unquestionably an abuse of power of the president."

Trump maintains that he did not suggest any "quid pro quo" in his call with Ukrainian President Volodymyr Zelensky, reportedly the subject of the whistleblower complaint, and said Tuesday that a "complete, fully declassified and unredacted transcript of my phone conversation" will be released Wednesday.

House Intelligence Committee chairman Adam Schiff also announced Tuesday that the whistleblower's counsel have been in touch with his committee, and that the whistleblower wants to testify before lawmakers.

"We're in touch with counsel and look forward to the whistleblower's testimony as soon as this week," Schiff tweeted. The whistleblower, whose identity remains unknown, is represented by Compass Rose Legal Group's Andrew Bakaj and fellow whistleblower attorney Mark Zaid.

The Senate Intelligence Committee is also reportedly seeking the testimony of the whistleblower and is also set to interview acting director of national intelligence Joseph Maguire and Intelligence Community Inspector General Michael Atkinson on Thursday, albeit separately.

The Republican-controlled U.S. Senate agreed by unanimous consent Tuesday to pass a nonbinding resolution calling for Trump administration officials to provide them with the whistleblower complaint.

Experts said the Ukrainian allegations pushing Democrats to impeachment doesn't necessarily mean the House won't pursue other legal claims against Trump. Democrats have previously called for the president's impeachment for obstructing justice during the course of Mueller's probe and are overseeing a multitude of other investigations into Trump's family, private businesses and his administration.

If the House does approve articles of impeachment, a trial will then be held in the Senate, where two-thirds of present senators are needed to remove a president from office. No GOP members of the Republican-controlled Senate have publicly backed impeachment.

Past impeachment inquiries have been led by the House Judiciary Committee, which has already been investigating whether to formally start such proceedings.

There was a similar spike in Democratic calls for impeachment after Mueller testified before Congress earlier this year about the findings made in his investigation on whether the Trump campaign colluded with Russia, and if the president obstructed justice during the course of the probe.

Mueller's report said the special counsel was unable to show that the Trump campaign and Russian officials knowingly conspired to help Trump win the 2016 election. But he declined to make a charging decision for Trump, citing an Office of Legal Counsel opinion that states a sitting president can't be indicted.

Experts said it's harder to prove that there was knowing intent of collusion when it involves a candidate, as in the case of the 2016-era allegations against Trump.

But they said the Ukrainian claims, which took place while Trump was in office, arguably constitute an abuse of power by the president.

Michael Gerhardt, a constitutional scholar at the University of North Carolina, said Trump's alleged actions largely revolve around instances of "self-dealing."

"The president has similarly done what Nixon had done," Gerhardt said. "But instead of asking domestic people to help" target his political opponents, "he's asking for foreign help to do that."

Both Gerhardt and Bowman said that the investigations into Trump aren't done yet. Bowman said that further details are needed surrounding the Ukrainian allegations in particular in order for an article of impeachment to be drafted based on those alone.

And other claims of misconduct against Trump may also make their way into the articles, whether in the form of a specific charge or in outlining broader allegations against the president.

Starting a formal impeachment inquiry may also bolster Democrats' chances at winning in court as they claim that some information, such as grand jury material redacted from the Mueller report, is necessary for the impeachment decision.

House Democrats are involved in several court cases seeking Trump's private financial records and other information relating to the president, and Pelosi has previously said she would await the outcome of those cases before making a decision on impeachment.

Still, Gerhardt underscored the significance of a sitting president allegedly seeking the help of a foreign power to investigate a political opponent.

"This ought to be a classic example of an impeachable offense," he said.

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