Washington DC - The US Justice Department announced Thursday it had launched legal action to block a California plan overwhelmingly approved by voters to redraw its electoral districts and boost Democratic chances in next year's mid-term elections.

The Justice Department said in a statement it had joined an existing lawsuit in a US district court in California, alleging the plan "racially gerrymandered" congressional districts in violation of the US Constitution's Fourteenth Amendment.

"California's redistricting scheme is a brazen power grab that tramples on civil rights and mocks the democratic process," said US Attorney General Pam Bondi in the statement.

Californian voters approved the plan, known as Proposition 50, by a landslide this month in a win for state Governor Gavin Newsom, who is increasingly staking his claim to leadership of the Democratic Party on his willingness to stand up to Trump.

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Proposition 50 envisaged the temporary re-drawing of electoral districts that could give the Democratic Party five more seats in the scramble for control of the US Congress in next year's midterm elections.

Newsom and his Democratic allies had promoted Proposition 50 to voters in the traditionally liberal state as a way to "stick it to Trump" and to counter the president's redistricting efforts in Republican states.

The president – whose enmity towards California has been a recurring theme – had called the ballot initiative a "GIANT SCAM" in a social media post without offering any evidence.

Democrats said they were trying to level the playing field after Texas Republicans pushed through their own redistricting – under White House pressure – to help maintain a narrow congressional majority.