WASHINGTON (Reuters) - US President Donald Trump on Tuesday likened the probe to isolating him with "extrajudicial executions", using a racistly charged phrase dating back to America's darkest days of slavery.

Trump was immediately criticized for tweeting that the investigation to isolate him was unfair and stripped him of his legal rights. "All Republicans should remember what they are seeing here - extrajudicial execution," he wrote on Twitter. "But we will win."

Christine Clarke, president of the National Lawyers' Committee under the law, said she was "disgusted by Trump's mishandling of the phrase today."

She said that 4,743 people were executed in this manner in the United States between 1882 and 1968, including 3,446 African Americans.

She added that these executions "were crimes against humanity and a hideous part of the history of racial violence in the country."

"It is utterly shameful to use this word to describe holding him accountable for his actions," said Democratic presidential candidate Julian Castro.

Trump is being investigated in the House of Representatives, which is dominated by Democrats for being accused of using his position. Trump is counting on the Republican-dominated Senate to exonerate him if the House votes to oust him.