Neither the White House nor senior Republicans reacted immediately.

In a 15-page opening statement quickly leaked to US media, Taylor recounted in deep detail how Ukraine’s new president, Volodymyr Zelensky, came under sustained pressure after taking office.

Taylor said he was told Trump wanted Zelensky “to state publicly” that he would investigate Biden and his son Hunter for what he said was corruption in the ex-Soviet republic.

He said that the US ambassador to the European Union, Gordon Sondland, told him that “’everything’ was dependent on such an announcement, including security assistance”. Another carrot being dangled, Taylor said, was a proposed summit between Zelensky and Trump.

Emerging from the testimony, Democratic congressman Adriano Espaillat tweeted that “what I heard today from #BillTaylor was very troubling and explosive”.

“At some point we’re going to have to conclude our investigation and go forward with what we have, and what we have is incredibly damning right now,” another Democratic legislator, Ted Lieu, said.

Democratic congresswoman Debbie Wasserman Schultz said: “It was the most damning testimony.”

She added: “I really felt like he drew the most direct line that I’ve heard from President Trump to the withholding of foreign aid to a vital diplomatic partner that needs those resources to keep Russia at bay – and tying the withholding of that funding and the meeting that President Zelensky wanted to President Zelensky declaring that he would open investigations into the Burisma-Biden issue and the 2016 election.”

Trump blocked Ukraine aid to force Kiev to probe Democrats, Mulvaney says

A new CNN/SSRS poll on Tuesday showed support for Trump’s removal from office is now 50 per cent cor to 43 per cent against.

However, reflecting the intensely partisan divide among voters, 87 per cent of Democrats are pro-impeachment, compared to just six per cent of Republicans.

Trump admits to asking Ukraine’s government to probe what he says is corruption related to Biden. However, he denies Democrats’ allegation that he tried to strong-arm the Ukrainians by linking the delivery of US military aid to his request for an investigation.

Trump has already branded the impeachment proceedings a “coup”, a “hoax”, a “witch hunt”, and a variety of swear words.

On Tuesday, he prompted an uproar by claiming he was the victim of a lynching, a word linked in the United States to the darkest days of slavery and its long aftermath.

“All Republicans must remember what they are witnessing here – a lynching,” Trump tweeted.

More than 3,400 African Americans were lynched between 1882 and 1968, and congressional Democrats reacted with collective anger.

“Lynching is a reprehensible stain on this nation’s history, as is this President. We’ll never erase the pain and trauma of lynching, and to invoke that torture to whitewash your own corruption is disgraceful,” Senator Kamala Harris, a Democratic presidential contender, wrote on Twitter.

Trump called her ‘bad news’. Now ex-Ukraine envoy is defying him to testify

The White House fought back, with spokesman Hogan Gidley insisting that Trump did not mean to compare “what’s happened to him to one of the country’s darkest moments”.

There was rare criticism from the top Republican legislator, Senator Mitch McConnell, who said Trump had made “an unfortunate choice of words”. He added, however, that Trump does have a “legitimate complaint” over a “totally unfair process”.

Trump will rely on McConnell to deliver an acquittal in the Senate if the lower house does go ahead with an impeachment vote.

The president did urge Zelensky in July to investigate Biden and his son, according to a summary of a phone call released by the White House.

Seeking help from a foreign country in a domestic election is illegal in itself, but a whistle-blower complaint about the call made the more serious allegation that Trump also sought to condition nearly US$400 million in military aid to Ukraine on Kiev launching a probe.

Taylor is a military veteran and career diplomat who served as US ambassador to Ukraine from 2006 to 2009, and is now charge d’affaires following the ouster of ambassador Marie Yovanovich this year.

Yovanovich testified to Congress two weeks ago, telling House investigators she was pushed out on “false claims” that she had disparaged Trump.

According to Taylor, much of the pressure he described was applied through “weird” backchannels, including Trump’s personal lawyer Rudy Giuliani.

Giuliani associates charged with illegally funnelling money to pro-Trump group

Taylor had already damaged Trump’s defence with a paper trail that included text messages in which he expresses alarm about having to pressure Zelensky’s government.

“Are we now saying that security assistance and [White House] meeting are conditioned on investigations?” Taylor asked Sondland in a September text message.

Later, Taylor expressed more explicit reservations to Sondland: “I think it’s crazy to withhold security assistance for help with a political campaign.”

On Tuesday, Taylor said in his opening statement that he stood by that characterisation of the quid pro quo being “crazy”.

Additional reporting by Reuters