Bribery, blackmail, extortion.

Call it what you will, but this is the new front being opened by the Democrats in their campaign to impeach Donald Trump.

At her weekly news conference at the Capitol, House Speaker Nancy Pelosi explicitly invoked bribery, which is identified as an impeachable offence in the US constitution.

"The devastating testimony corroborated evidence of bribery uncovered in the inquiry," she said, referring to the first of a series of open impeachment hearings being conducted by Democrat-led committees as part of the impeachment investigation.

"And that the President abused his power and violated his oath by threatening to withhold military aid and a White House meeting in exchange for an investigation into his political rival — a clear attempt by the President to give himself an advantage in the 2020 election."

What's clear is that Democrats are trying to sharpen the charge against the President as they attempt to advance the process to drafting articles of impeachment, with a possible House vote to impeach before Christmas.

"The bribe is to grant or withhold military assistance in return for a public statement of a fake investigation into the elections. That's bribery," Ms Pelosi added later.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 15 seconds 1 m 15 s Acting US Ambassador to Ukraine, William Taylor, testifies in impeachment case against Donald Trump

Others have described the alleged act as blackmail, a threat to withdraw military aid that Ukraine had come to rely on to resist Russian attacks on its territory.

"If we find that the President of the United States abused his power … or if he sought to condition, coerce, extort, or bribe an ally into conducting investigations to aid his re-election campaign and did so by withholding official acts — a White House meeting or hundreds of millions of dollars of needed military aid — must we simply 'get over it?'" Democratic committee chairman Adam Schiff said as he opened the hearing.

The deeper question is whether the act is impeachable, under the constitutional definition for treason, bribery, or other high crimes and misdemeanours.

Republicans think the whole thing is just a witch hunt.

"It seems you agreed wittingly or unwittingly to participate in a drama, the main performance, a Russia hoax has ended, and you've been cast in the low-rent Ukrainian sequel," Republican Devin Nunes told the first two witnesses.

What is this actually about again?

Republican Devin Nunes called the proceedings a "low-rent Ukrainian sequel" to the Russia probe. ( AP: Saul Loeb )

It was the first impeachment hearing since Bill Clinton was impeached in 1998, yet in a country now desensitised to chaos it didn't really grip the nation.

There was a queue, sure, and it was televised, but somehow James Comey's testimony to Congress and Special Counsel Bob Mueller's had a bigger sense of occasion.

At the centre of the inquiry is the allegation that Mr Trump withheld military aid to Ukraine (and the prospect of a White House meeting with President Volodymyr Zelensky) to try to force the Ukrainian Government to investigate the business activities of Democrat Joe Biden's son Hunter, who was once on the board of a Ukrainian company called Burisma.

The premise is that such an investigation would smear the Bidens to the extent that it would help the President's 2020 re-election prospects.

The facts are that Mr Trump did ask the Ukrainian President to conduct such an investigation, and that military aid was withheld for more than 50 days, and only released after news of the hold started leaking out.

The White House has argued that the President was testing the new Ukrainian administration before handing out yet more US cash.

More than $US1.6 billion in military aid has been extended since pro-Russian separatists moved into Eastern Ukraine in 2014.

The first two witnesses to appear publicly in the inquiry, State Department Ukraine expert George Dent and acting US Ambassador to Ukraine Bill Taylor, both outlined their deep commitment to effective foreign policy in regard to the country.

"Ukraine is a strategic partner of the United States, important for the security of our country as well as Europe," Mr Taylor told the hearing.

"Ukraine is on the front line in the conflict with a newly aggressive Russia."

"In July 2014, these Russian-led forces in Donbas," he said of the conflict, "shot down a civilian airliner en route from Amsterdam to Malaysia, killing all 298 people on board."

It was the shooting down of MH17, Mr Taylor explained, that cemented the commitment to military aid to counter the Russians. (I know, Russia again.)

He described his astonishment when he learnt that aid was on hold and a cable he sent to Secretary of State Mike Pompeo suggesting that playing into the hands of the Russians was "folly".

After 38 Australians died on MH17, Australia also opened an embassy in Ukraine and has since committed substantial aid and deepened dialogue with Ukraine, complementing the US approach.

One thing that was clear during the hearing was the increasing alarm felt by those who built that policy framework.

"We, the Europeans, and most of the West imposed economic sanctions and kicked Russia out of the G8," he said.

"Beginning in 2014, we and NATO began to provide military assistance to Ukraine's armed forces."

It's the kind of foreign policy detail that will go over the heads of most Americans, even coupled with the discomfort that the baritone-voiced Mr Taylor expressed about "an irregular, informal channel of US policy-making with respect to Ukraine, unaccountable to Congress".

He was talking about the channel being run by Mr Trump's personal lawyer Rudy Giuliani: "In an instant I realized that one of the key pillars of our strong support for Ukraine was threatened. The irregular policy channel was running contrary to the goals of longstanding US policy."

Impeachment views split along party lines

Ambassador Bill Taylor said it was the shooting down of MH17 that cemented the commitment to military aid. ( Reuters: Jonathan Ernst )

The population is almost evenly split on impeachment.

Those in the room doing the questioning formed their partisan opinions long ago.

It's either a noble pursuit, according to Democrats, or a vile hoax, according to Republicans, who say the hearings are all for show, given that those giving evidence have already been deposed behind closed doors.

Much of what was said in the open stuck to that script.

However, in the wake of the immense Mueller Report, Democrats are aware that most Americans won't have bothered to read the written transcripts. They want the witnesses on screen.

Republicans have pointed out that much of the testimony from the first two witnesses was based on hearsay.

Democrats have responded that those who have had direct contact with the President, like former National Security Adviser John Bolton and Chief of Staff Mick Mulvaney, have refused to appear.

There'll be at least another week of hearings before Democrats move to draft articles of impeachment ahead of a vote.

Expect to hear the word "bribery" a lot, because Democrats will have to prove that what Donald Trump did wasn't just irregular folly, but that he broke the law.