WASHINGTON - Speaker of the House Nancy Pelosi announced the opening of a formal impeachment inquiry into United States President Donald Trump's conduct on Tuesday (Sept 24) on the grounds that he betrayed his oath of office by seeking the help of a foreign power to dig up dirt on a political rival.

"Today, I'm announcing 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."

The President immediately slammed the inquiry as another "witch hunt".

The inquiry, a necessary first step in what could be a long impeachment process with no guaranteed outcome, comes after the revelation of a July 25 phone call in which President Trump allegedly urged Ukrainian President Volodymyr Zelensky to investigate former US vice-president and Democratic Party front runner Joe Biden's son, Hunter Biden.

Mr Trump is accused of withholding promised American aid to Ukraine to lean on Mr Zelensky to investigate the younger Biden. He has denied the charge and insisted that there was nothing inappropriate about the call.

The call was exposed by an as-yet-unnamed whistle-blower in the intelligence community.

Until Tuesday, many Democrats were reluctant to launch impeachment proceedings, worried that the political backlash would further solidify and inflame President Trump's supporters.

But the mood turned on Tuesday, even as President Trump attempted to get ahead of the tide, telling reporters that the full, un-redacted transcript of the phone call would be released on Wednesday.

Ms Pelosi said the President had admitted asking the President of Ukraine to take actions that would benefit him politically. This was a betrayal of the oath of office, and of national security, she charged.

Later, she told journalists: "It's truly a sad day for our country."

A formal impeachment inquiry is the first step towards actual impeachment, but does not necessarily mean the process will end with impeachment.

If the inquiry does produce a motion to impeachment, the House would need a simple majority to send it to the Senate which would essentially put the accused on trial.

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 an unlikely scenario - to finally impeach President Trump.

The President who is in New York City for the United Nations General Assembly, reacted almost immediately, tweeting: "Such an important day at the United Nations, so much work and so much success, and the Democrats purposely had to ruin and demean it with more breaking news Witch Hunt garbage. So bad for our Country!"

Earlier, as news of the impending announcement spread, he told reporters: "Our country's doing the best it's ever done and they're going to lose the election and they figured this is a thing to do.

"If she (Ms Pelosi) does that, they all say that's a positive for me from the election," he said.

"The good news is the voters get it. This is why they say it's good for the election."

While impeachment is unlikely, the process of the inquiry itself gives Congress wide powers and could be damaging to the President. Much depends on the contents of the call as per the transcript, which will be the next act in the drama.

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

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

The whistle-blower has reportedly said there were multiple contacts with the President of Ukraine, so whatever emerges from the transcript could add fuel and lead to demands for more transcripts and related documents.

Earlier on Tuesday, the President said he had withheld almost US$400 million (S$550 million) in aid pledged to Ukraine - which has been tied to the call - because he had wanted Europe to do more for the Ukraine, which faces an expansionist Russia.

The money was eventually paid when lawmakers expressed concerns.

"Those funds were paid," he said. "They were fully paid but my complaint has always been - and I'll withhold again and I'll continue to withhold until such time as Europe and other nations contribute to Ukraine. Because they're not doing it."

Later he complained to reporters: "Why is it always the United States that pays?"

Previously, the President said he had withheld the aid because of concerns over corruption in Ukraine. And he doubled down on calls to investigate Mr Joe 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 was involved.

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

Mr Brad Parscale, the Trump 2020 campaign manager, said in an e-mail statement: "Democrats can't beat President Trump on his policies or his stellar record of accomplishment, 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."

Launching an impeachment inquiry 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."