US President Donald Trump on Tuesday slammed the impeachment inquiry as a "lynching," using explosive, racially-charged language to discredit the probe as a key witness who expressed concerns about political interference testified to Congress.

Trump has relentlessly attacked the investigation as part of a broader partisan "witch hunt" against him.

But on Tuesday he used particularly inflammatory language to lay into his opponents, saying they were seeking to impeach him "without due process or fairness or any legal rights."

"All Republicans must remember what they are witnessing here - a lynching," Trump raged, using a word that evokes the darkest days of America's slavery legacy.

More than 3,400 African Americans were lynched between 1882 and 1968, and congressional Democrats, many of them black, reacted with collective revulsion.

"How dare you?!" seethed congresswoman Barbara Lee on Twitter as she assailed the "disgusting and ignorant message" from Trump.

"That is one word that no president ought to apply to himself," House Majority Whip James Clyburn, the most senior black lawmaker in Congress, told CNN.

"I'm a product of the South. I know the history of that word. That is a word that we ought to be very, very careful about using," the South Carolina Democrat said.

Few Republican lawmakers have openly criticized Trump for his comment, but Senator Susan Collins, a moderate Republican and occasional Trump critic who is up for re-election in 2020, spoke out. "'Lynching' brings back images of a terrible time in our nation's history, and the President never should have made that comparison," she tweeted.

Despite the White House's decision to not cooperate with the probe, Democrats have pressed on, seizing a victory of sorts with the closed-door testimony Tuesday of Bill Taylor, the acting ambassador to Ukraine.

His communications with other diplomats raised red flags about Trump's pressure campaign against Kiev that is at the heart of the Democrats' investigation.

Trump urged his Ukrainian counterpart Volodymyr Zelensky in July to investigate the US leader's Democratic rival Joe 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 whistleblower complaint about the call made the more serious allegation that Trump also sought to condition nearly $400 million in military aid to Ukraine on such probes.

Democrats have described such a quid pro quo, and the seeking of foreign interference in US elections, as impeachable offenses and have intensified their probe of the president.

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.

Taylor could be one of the most consequential investigation witnesses to date.

In text messages turned over to Congress, he expressed serious reservations about helping coordinate the Trump pressure campaign against Ukraine.

Taylor, US Ambassador to the European Union Gordon Sondland and other top US diplomats discussed leveraging a potential summit between Trump and Zelensky on a pledge to investigate 2016 election interference and a company that employed the son of Biden, a frontrunner in the 2020 Democratic nomination race.

"Are we now saying that security assistance and WH (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."

Democrats say they hope Taylor will help fill gaps in the text messaging between the diplomats and shed light on the pressure campaign.

"I think he's going to be forthcoming," Democratic congressman Ro Khanna, who as a member of the House Oversight Committee has sat in on multiple witness depositions, told CNN.

Taylor "really can help shed light on whether our policy towards Ukraine has been driven by American national interests, or whether the president has hijacked it for his own re-election in 2020," Khanna said.

One fresh source of irritation for the White House is a new CNN poll released Tuesday showing 50 percent of Americans now support impeachment and removal from office, a new high in the network's polling, against 43 percent who are opposed. Trump is working to ensure the unfailing support of Republican lawmakers, repeating on Twitter his frequent exaggeration that he has "95% Approval Rating in the Republican Party."

Copyright Agence France-Presse, 2019