Democratic groups and party leaders in the United States have filed a lawsuit to block an executive order by US President Donald Trump seeking to crack down on mail-in voting, saying he tried to rewrite election rules to his advantage.

Trump's order, signed on Tuesday, would restrict a popular way of casting ballots, following the Republican president's repeated attacks – based on disproven conspiracy theories – about supposed cheating by Democrats.

The 64-page complaint filed on Wednesday by the Democratic National Committee and other party organisations alleged that changes outlined in Trump's order "imminently threaten to disenfranchise lawful voters and plainly exceed the President's lawful authority."

"(The executive order) dramatically restricts the ability of Americans to vote by mail, impinging on traditional state authority...It is an unlawful exercise of authority that must be declared invalid," the complaint read.

Polls show that the Republican Party faces a serious threat of losing its narrow control of Congress in the midterm elections in November, particularly in the House of Representatives.

If Democrats win, they have signalled they will block Trump's agenda and could move to impeach him.

Trump's order limits mail-in ballots only to people on "State Citizenship Lists" compiled by his administration – which Democrats say is an attempt to "disenfranchise voters for partisan advantage."

Trump has been pressing hard for measures that experts believe would likely lower turnout, with potentially huge consequences in parts of the country where elections are decided by razor-thin margins.

His push for federal election overhauls has run up against constitutional concerns.

Under the US Constitution, states retain broad control over the administration of elections.

Trump's executive order came as the Republican Party failed to pass a more far-reaching set of voting restrictions called the "SAVE America" act.

The Democratic Party's lawsuit filed in a Washington, DC federal court also names Senate Minority Leader Chuck Schumer and House Minority Leader Hakeem Jeffries as plaintiffs. (AFP)



Edited by Robert Kemp