Donald Trump speaks in the Oval Office of the White House in Washington

Donald Trump has doubled down on his vicious tweets attacking black congressman Elijah Cummings by retweeting far-right commentator Katie Hopkins and calling Mr Cummings "racist".

The US president described Mr Cummings' Baltimore district as a “filthy” and “rat and rodent infested mess” where “no human being would want to live”.

It comes after the US House judiciary committee asked a judge to force the release of grand jury evidence from the Mueller investigation, as four Washington members of Congress called for Mr Trump’s impeachment.

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The request, filed on Friday, explicitly referenced congress’ impeachment powers and is a major step forward in the Democrats' legal fight against Mr Trump.

California Governor Gavin Newsom is also reportedly on the verge of a move that could force Mr Trump to release his tax returns if he wants to appear on the state’s ballot in the 2020 elections.

Meanwhile, political commentators have noted that Mr Trump has used the word "infested" six times on his Twitter account - and each time it has been in reference to areas populated predominantly by people of colour.

His attacks directed at Mr Cummings also appeared to have been inspired by a Fox & Friends report that aired minutes before the president’s outburst, which criticised living conditions in the Democratic congressman's Baltimore district.