The Republican National Committee and two other Republican groups have filed a lawsuit against California to stop the state from mailing absentee ballots to all voters ahead of the 2020 general election.

Democrat Governor Gavin Newsom decided to encourage mail-in voting, specifically for November’s presidential election, as part of the state’s response to the coronavirus pandemic.

The RNC is challenging the move by the country’s most populous state, making the suit a significant front in the battle between Republicans and Democrats over the issue of mail-in votes. Similar legal challenges are ongoing in approximately a dozen states.

Donald Trump has been particularly vocal in his opposition to mail-in votes, claiming there is widespread fraud when they are used, but without providing evidence.

RNC Chairwoman Ronna McDaniel accused Democrats of using the pandemic as an excuse to implement a partisan election agenda, describing the governor’s order as “the latest direct assault on the integrity of our elections”.

“Newsom's illegal power grab is a recipe for disaster that would destroy the confidence Californians deserve to have in the security of their vote,” she said.

On Sunday the president accused Democrats of trying to rig the 2020 election. He tweeted: “The United States cannot have all Mail In Ballots. It will be the greatest Rigged Election in history.”

He continued: “People grab them from mailboxes, print thousands of forgeries and 'force' people to sign. Also, forge names. Some absentee OK, when necessary. Trying to use Covid for this Scam!”

There is no evidence to support the claim that “thousands” of forgeries are linked to vote-by-mail. Nor is there evidence of voters being coerced into signing absentee ballots.

The president and his family have voted by mail in at least three elections since he took office. In an exchange with a reporter in April, Mr Trump described voting by mail as horrible and corrupt. Attempting to reconcile the facts with his position, he said: “Sure I can vote by mail ... Because I'm allowed to.”

Condemning Democrats' efforts to strengthen mail-in voting during discussions over a coronavirus stimulus bill in April, Mr Trump told Fox News that if everyone entitled to vote was allowed to "you'd never have a Republican elected in this country again".

Joining the RNC on the California lawsuit are the National Republican Congressional Committee and the California Republican Party.

The suit argues that the governor’s order “violates eligible citizens' right to vote” and that it will lead to electoral fraud as inactive voters will also be mailed absentee ballots which may be misused.

Responding to the suit, Governor Newsom’s press secretary Jesse Melgar said in a statement: “California will continue to defend Californians' right to vote, including their right to vote by mail, and the right to hold an election that is safe, secure, and accessible. Voters shouldn't have to choose between their health and their right to vote.”

No evidence of widespread voter fraud has been found in studies of both in-person or mail-in voting.

Mr Trump’s own advisory commission on election integrity spent eight months, from May 2017 to January 2018, probing claims of voter fraud and did not turn up a single confirmed instance.

A Washington Post review of data from after the 2016 election found just four confirmed cases of voter fraud: three people who tried to vote for Mr Trump twice — and were caught — and an election worker in Miami who was caught trying to fill in a bubble on someone else’s ballot for a local mayoral candidate.

House Democrats passed a $3trn coronavirus response bill earlier this month that includes $3.6bn for election security, including expanded access to mail-in voting.

The bill is expected to languish in the Republican-controlled Senate.