US President Donald Trump has called the congressional impeachment investigation that may seek to remove him from office "a lynching".
In a tweet Trump condemned the inquiry as "without due process or fairness or any legal rights".Democrats may seek to impeach the Republican president by Christmas.
A racially loaded term in the US, lynching refers to historic extrajudicial executions by white mobs mainly against African Americans.
What's the reaction?
Trump has repeatedly called the impeachment inquiry, as well as an earlier investigation into foreign meddling in the 2016 election, a "witch hunt".
But his likening of a process enshrined in the US constitution to racist killings provoked outrage among African-American lawmakers.
Congressman Jim Clyburn told CNN on Tuesday: "That is one word that no president ought to apply to himself."
"I've studied presidential history quite a bit and I don't know if we've ever seen anything quite like this.""I'm a product of the South," the South Carolina Democrat added. "I know the history of that word. That is a word that we ought to be very, very careful about using."
Bobby Rush, an Illinois Democrat, tweeted to Mr Trump: "What the hell is wrong with you?"
"Do you know how many people who look like me have been lynched, since the inception of this country, by people who look like you."
He called on Trump to delete the tweet.
What's the latest with the impeachment inquiry?
The inquiry is examining whether the Republican president abused his office by improperly pressuring Ukraine to launch an investigation into former US Vice-President Joe Biden, a leading candidate for the Democratic 2020 presidential nomination.
Trump denies holding up US military aid to Ukraine so they would investigate Mr Biden's son, who worked for a Ukrainian gas company.
On Tuesday, veteran US diplomat William Taylor - the acting US ambassador to Ukraine - is scheduled to be interviewed by the impeachment committees at Congress.
Texts show Mr Taylor raised the alarm to other Trump officials about withholding US aid to Ukraine.
"I think it's crazy to withhold security assistance for help with a political campaign," Mr Taylor wrote in one message.
During a cabinet meeting at the White House on Monday, Mr Trump called for his party "to get tougher and fight" the impeachment inquiry.
"We have some that are great fighters, but they have to get tougher and fight, because the Democrats are trying to hurt the Republican party before the election," he said.
Initially hopeful the impeachment inquiry would be finished by November's Thanksgiving holiday, Democrats are now signalling it may drag on towards Christmas.
Quick facts on impeachment
Impeachment is the first part - the charges - of a two-stage political process by which Congress can remove a president from office
If the House of Representatives votes to pass articles of impeachment, the Senate is forced to hold a trial
A Senate vote requires a two-thirds majority to convict - unlikely in this case, given that Mr Trump's party controls the chamber
Only two US presidents in history - Bill Clinton and Andrew Johnson - have been impeached but neither was convicted and removed
President Nixon resigned before he could have been impeached