WASHINGTON (REUTERS) - As another key career diplomat witness appeared on Tuesday (Oct 22) in the impeachment inquiry into United States President Donald Trump's dealings with Ukraine, Mr Trump called himself the victim of a "lynching," a comment that dredged up a painful chapter in race relations and was swiftly condemned by numerous lawmakers.

Mr William Taylor, who as the charge d'affaires is the top US envoy in Ukraine, walked past journalists without answering questions as he made his way into the US Capitol for closed-door testimony to the three Democratic-led House of Representatives committees leading the inquiry.

His appearance marks another pivotal development in the political drama unfolding in Washington - focusing on Republican Trump's request to Ukraine to investigate Democratic political rival Joe Biden - that threatens Mr Trump's presidency even as he pursues re-election in 2020.

Mr Trump, a day after calling on his fellow Republicans to get tougher in defending him in the inquiry, inflamed the controversy by writing on Twitter: "All Republicans must remember what they are witnessing here - a lynching. But we will WIN!"

African American lawmakers and others denounced Mr Trump for the remark because of the past US history of lynching of black people, particularly in formerly pro-slavery Southern states.

"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.

But Republican Senator Lindsey Graham, whose home state of South Carolina has a large black population, defended Mr Trump's language, saying that "this is a lynching in every sense. This is un-American".

The Trump administration has not cooperated in the impeachment inquiry, seeking to block testimony and documents. The House Intelligence Committee issued a subpoena to compel Mr Taylor's testimony after the State Department had directed him not to appear, an official involved in the inquiry said.

The House is focusing on Mr Trump's request during a July 25 telephone call with Ukrainian President Volodymyr Zelensky that he investigate former vice-president Biden and his son Hunter. The elder Mr Biden is a leading contender for the 2020 Democratic presidential nomination to face Mr Trump. Mr Hunter Biden had served on the board of a Ukrainian energy company.

Mr Trump made his request - described by Democrats as an improper invitation for foreign interference in an American election - after withholding US$391 million (S$533 million) in security aid approved by the US Congress to help combat Russia-backed separatists in eastern Ukraine. Mr Zelensky agreed to the request.

The aid was later released.

Mr Taylor mentioned his concern about withholding US aid on Sept 9 to Mr Kurt Volker, the State Department's then special envoy to Ukraine, and Mr Gordon Sondland, the US ambassador to the European Union, in a text message provided to investigators and later made public.

"As I said on the phone, I think it's crazy to withhold security assistance for help with a political campaign," Mr Taylor wrote.

Mr Taylor was tapped to serve as charge d'affaires in Kiev, where he served as US ambassador from 2006 to 2009, after Mr Trump in May abruptly removed Ambassador Marie Yovanovitch.

Mr Trump's personal lawyer Rudy Giuliani had portrayed her as resisting his efforts to push Ukraine to investigate the Bidens.

Ms Yovanovitch testified in the impeachment inquiry on Oct 11.

Mr Trump has denied any wrongdoing and accused Democrats of trying to oust him to prevent him from being re-elected.

REPUBLICAN RESPONSE

Mr Trump's lynching remark was his latest racially tinged comment, coming two years after he said there were "very fine people on both sides" after clashes at a rally by white nationalists in Charlottesville, Virginia.

Mr Hakeem Jeffries, chairman of the House Democratic caucus, urged Mr Trump to apologise for his lynching comment.

Some Republicans denounced Mr Trump's language.

Senator Susan Collins wrote on Twitter: "'Lynching' brings back images of a terrible time in our nation's history, and the President never should have made that comparison."

Republican congressman Adam Kinzinger called on Mr Trump to retract the remark, writing on Twitter: "We can all disagree on the process, and argue merits. But never should we use terms like 'lynching' here. The painful scourge in our history has no comparison to politics."

Mr Tim Scott, the sole black Republican in the Senate who like Mr Graham represents South Carolina, told reporters he understands Mr Trump likening the impeachment inquiry to a "death row trial", but added: "I wouldn't use the word lynching."

Similarly, House Minority Leader Kevin McCarthy told reporters, "That's not the language I would use."

If the Democratic-led House approves of articles of impeachment - formal charges - against Mr Trump, the Republican-led Senate would then hold a trial on whether to remove Mr Trump from office. Few Republicans have shown an inclination to remove him.