Washington DC - Democratic groups and party leaders have filed a lawsuit to block an executive order by President Donald Trump seeking to severely restrict mail-in voting.

Trump's order, signed Tuesday, would massively curtail a popular way of casting ballots, following completely baseless claims of electoral fraud pushed by the 79-year-old and his allies.

The complaint filed Wednesday by the Democratic National Committee and other party organizations 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.

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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 signaled 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."

Republicans have been pushing 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.

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

Trumps executive order came as the GOP failed to pass an even farther-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.