Democratic Chairman of the House Intelligence Committee Adam Schiff and the panel's top Republican, Devin Nunes, have each given scathing opening statements.

Schiff introduces the witnesses, Fiona Hill and David Holmes, who he says were "shocked" when they discovered that the White House was requesting that Ukraine investigate Trump's political rival.

He says if Trump tried to "condition, coerce, extort or bribe a vulnerable ally... it will be up to us to decide, whether those acts are compatible with the office of the presidency".

Nunes says Democrats are on a "carousel of accusations" and can't make up their mind in these "bizarre hearings" whether they are accusing Trump of bribery, extortion, obstruction of justice or some other impeachable offence.

"There's no actual misdeed and no one even claiming to be a victim," says Nunes.

Democrats, he says, "have poisoned the minds of fanatics", who oppose Trump. He repeatedly says Democrats "got caught" peddling in conspiracy theories.

Image source, JIM WATSON/AFP via Getty Images

The witness will also fire off an implicit rebuke of President Trump.

"If the President, or anyone else, impedes or subverts the national security of the United States in order to further domestic political or personal interests, that is more than worthy of your attention," Hill will say in her seven-page statement.

"But we must not let domestic politics stop us from defending ourselves against the foreign powers who truly wish us harm."

Hill will say downplaying alleged Russian election interference has far-reaching consequences for American democracy.

"These fictions are harmful even if they are deployed for purely domestic political purposes," Hill will say.

"When we are consumed by partisan rancour," she will continue, "we cannot combat these external forces as they seek to divide us against each another, degrade our institutions, and destroy the faith of the American people in our democracy."

She will say Putin and his team "operate like a Super PAC", spending millions to weaponise US opposition research and false narratives.

(A PAC is a Political Action Committee - if you need a refresher on how these groups influence US politics, we've explained it here and here.)

Image source, Chip Somodevilla/Getty Images

Also according to her prepared remarks, Hill will say the result of Russia's alleged campaign is that "our nation is being torn apart".

"Truth is questioned," she will say. "Our highly professional and expert career foreign service is being undermined. US support for Ukraine - which continues to face armed Russian aggression - has been politicised."

She will add: "The Russian government’s goal is to weaken our country - to diminish America’s global role and to neutralise a perceived US threat to Russian interests."

Hill will say she's bringing all this up "not as an alarmist but as a realist".

"I refuse to be part of an effort to legitimise an alternate narrative that the Ukrainian government is a US adversary, and that Ukraine - not Russia - attacked us in 2016."

Image source, BRENDAN SMIALOWSKI/AFP via Getty Images

Fiona Hill will rebuke lawmakers on the panel before her as she warns them against promoting falsehoods that minimise Russia's alleged attempts to interfere in US elections.

According to her prepared remarks, which we will hear in a few moments, she will say: "Some of you on this committee appear to believe that Russia and its security services did not conduct a campaign against our country - and that perhaps, somehow, for some reason, Ukraine did.

"This is a fictional narrative that has been perpetrated and propagated by the Russian security services themselves.

"The unfortunate truth is that Russia was the foreign power that systematically attacked our democratic institutions in 2016.

"This is the public conclusion of our intelligence agencies, confirmed in bipartisan Congressional reports.

"It is beyond dispute, even if some of the underlying details must remain classified."

Here's a refresher on the faces to know in the hearing room today - the two top lawmakers and their party's counsel.

Image source, Getty Images

Holmes is a political counselor in the US embassy in Kyiv, who says he overheard a conversation between EU Ambassador Gordon Sondland and Trump on 26 July.

The call, which took place in a Kyiv restaurant, occurred one day after Trump spoke to Ukrainian President Volodymyr Zelensky, urging him to investigate Biden.

"So, he's gonna do the investigation?" Holmes says he heard Trump ask Sondland.

Holmes says he was also told by Sondland that Trump did not "give a [expletive] about Ukraine", and only cares about "big stuff" like the "Biden investigation".

Image source, Getty Images

Fiona Hill was a top Russia expert at the White House who quit days before the 25 July Trump-Zelensky call that triggered the impeachment hearings.

Hill was born in County Durham, England, and is a former US intelligence official specialising in Russia. She has served under three US presidents.

A coal-miner's daughter, Hill married an American she met at Harvard and holds dual nationality.

She previously testified that she was present at a 10 July meeting in the White House with Ukrainian officials where EU Ambassador Gordon Sondland promised a meeting between President Trump and Ukrainian President Volodymyr Zelensky in exchange for "investigations" into Biden.

She said ex-national security adviser John Bolton cut the meeting short and told her "I am not part of whatever drug deal Sondland and [Acting White House Chief of Staff Mick] Mulvaney are cooking up on this", and to report it to White House lawyers.

Hill, who has been assailed by right-wing media, said in a closed-door deposition earlier this month that she had received death threats.

This is the fifth day of public hearings in the impeachment investigation into President Donald Trump.

Today, lawmakers will hear from Fiona Hill, British-born top Russia expert at the White House who resigned in July days before the Trump-Ukraine call that triggered this congressional investigation.

A former intelligence official, Hill's testimony will confront unsubstantiated theories of Ukrainian meddling in the 2016 election that have been repeatedly raised by Republicans during the impeachment inquiry.

Also testifying is David Holmes, who works at the US embassy in Kyiv. He will be asked about a phone conversation he says he overheard in a restaurant in Ukraine's capital in which President Donald Trump asked an envoy about certain investigations.

The question at the heart of this impeachment inquiry remains: did President Donald Trump abuse his power by pressuring Ukraine to investigate his political rival, Joe Biden? The inquiry looks likely to culminate in a congressional vote as to whether the president should be removed in office

We’ll keep you updated on all the latest from Capitol Hill here.