WASHINGTON - US President Donald Trump lashed out at the Democrats after the opposition party announced a formal impeachment inquiry, signalling a long and bruising gloves-off battle going into the 2020 election season.

"The Democrats are frozen with hatred and fear," the President tweeted on Wednesday (Sept 25). "They get nothing done. This should never be allowed to happen to another President. Witch Hunt!"

Earlier on Tuesday, Speaker of the House Nancy Pelosi said President Trump's conduct had violated his oath of office and duty under the Constitution. The trigger - an unnamed whistle-blower's complaint that suggested the President sought a quid pro quo from Ukraine's President Volodymyr Zelensky that would damage his potential 2020 rival, former vice-president Joe Biden, in return for aid.

Ms Pelosi accused the President of betraying his oath of office.

"The House of Representatives is moving forward with an official impeachment inquiry," she told a press conference. "I'm directing our six committees to proceed with their investigations under that umbrella... The President must be held accountable."

Washington was waiting for the release on Wednesday of a transcript of the controversial July phone call Mr Trump had with President Zelensky.

But questions were already being raised as to whether it would be a transcript or just notes on the call. And given the whistle-blower has reportedly said there were multiple contacts with Ukraine, whatever emerges will likely just fuel more demands for transcripts and related documents and testimony.

Republican Senator Marco Rubio tweeted: "Impeachment is an extraordinary measure that should be employed with caution & on the basis of facts. It nullifies an election & is deeply divisive & disruptive. And yet many didn't even wait to read the transcripts or the complaint before making up their minds."

Until Tuesday, many Democrats were reluctant to launch impeachment proceedings, worried that the political backlash would further solidify and inflame Mr Trump's base. But the mood turned as Mr Trump appeared to admit that Mr Biden's name had figured in the phone call, even as he denied any quid pro quo offer.

He had stalled almost US$400 million (S$551 million) in aid to Ukraine because he wanted Europe to do more for Ukraine first, he told reporters in New York where he has been attending the United Nations General Assembly. "Why is it always the United States that pays?" he said.

Mr Trump has also doubled down on calls to investigate Mr Biden over old allegations that he had used the influence of his office in 2016 to persuade Ukraine to fire a prosecutor who was investigating a company, Burisma Holdings, in which his son, Hunter Biden, was involved.

The prosecutor was fired, but the allegations against Mr Biden came to nothing.

The impeachment inquiry is a first step in a long process with no guaranteed outcome.

If it does produce a motion to impeach, the House would need a simple majority to send it to the Senate. But impeachment needs a two-thirds majority in the Senate, which is controlled by the Republican Party. It would need significant desertions by Republicans - currently a highly unlikely scenario - to finally impeach Mr Trump.

Yet while impeachment is likely to fail ultimately, the process of the inquiry itself gives Congress wide powers.

"This will make it much more difficult for the Trump administration to refuse to hand over documents or refuse to allow people to testify," Dr Glenn Altschuler, professor of American Studies at Cornell University, told The Straits Times.

One of the key reasons why Democrats decided to launch a formal inquiry, was frustration with the stonewalling tactics of the White House, he said. Now if the administration goes to the courts, the courts would in all likelihood side with Congress.

Brad Parscale, the Trump 2020 campaign manager, in an e-mail said: "Democrats can't beat President Trump on his policies or his stellar record… so they're trying to turn a Joe Biden scandal into a Trump problem.

"The misguided Democrat impeachment strategy is meant to appease their rabid, extreme, leftist base, but will only serve to embolden and energise President Trump's supporters and create a landslide victory for the President."

It is certainly a high risk strategy for the Democratic Party, Dr Altschuler said. "But what is important is that this gives the Democrats an issue that looks ahead to 2020 and interference in the election, and that will resonate with voters."