US President Donald Trump on Tuesday likened the impeachment investigation against him to a "lynching," a racially charged word from the darkest days of America's slavery legacy.

There was an immediate backlash to Trump's tweet in which he claimed that impeachment was unfair and stripping him of his legal rights, although a senior Republican senator backed up the president.

"All Republicans must remember what they are witnessing here -- a lynching. But we will WIN!," Trump tweeted.

Kristen Clarke, president of the Lawyers' Committee for Civil Rights Under Law, said she was "sickened to see Trump's gross misappropriation of this term today."

Clarke said that 4,743 people were lynched in the United States between 1882 and 1968, of which 3,446 were African Americans.

"Lynchings were crimes against humanity and an ugly part of our nation's history of racial violence," she said.

"It's beyond shameful to use the word 'lynching' to describe being held accountable for your actions," said Democratic presidential candidate Julian Castro.

Trump is being investigated for possible impeachment in the Democratic-led House of Representatives over his alleged abuse of office. He is counting on the Republican-dominated Senate to acquit him if the House does vote to impeach.

Senator Lindsey Graham, an influential Republican, however, said he fully agreed with Trump's complaint.

"Yes this is a lynching in every sense. This is un-American," he told reporters.

Graham said that the impeachment probe failed to give Trump the ability to defend himself and is a "complete sham."

"I will do everything I can to make sure it doesn't live very long in the Senate," he said.