Donald Trump has alleged without evidence that Democrats are cheating in California’s primaries and claimed in a late-night social media post that the US attorney’s office in Los Angeles was investigating.

As counting continues in the most populous state in the US, the president’s unfounded remarks are likely to further alarm election observers, who have warned of the risk of escalating misinformation in the absence of a final result.

Trump has a history of undermining election results that don’t go in his favor. He has repeatedly alleged that Democrats “stole” the 2020 presidential election, which he lost to Joe Biden, despite privately admitting his defeat, according to aides.

At 12.48am on Thursday, Trump posted: “The Dumocrats are at it again! They are trying to STEAL THE GOVERNOR OF CALIFORNIA PRIMARY, AND THE MAYOR OF LOS ANGELES, PRIMARY, AWAY FROM TWO GREAT REPUBLICAN CANDIDATES. Here we go with the very late and massive numbers of MAIL IN BALLOTS.”

“There’s BIG cheating by the Dumocrats in California,” he said 17 minutes later in another post on his Truth Social platform. “Votes are all tied up. May not be in for weeks. Under investigation by the U.S. Attorney’s Office in Los Angeles. Why the vote counting DELAY??? President DJT”.

The US attorney’s office said it had no comment on Trump’s claim that his allegations of cheating are “under investigation” by US attorneys. The Department of Justice in Washington DC did not immediately respond to a request for comment.

The president presented no substantive basis for questioning the legitimacy of the election.

Mail-in ballots factor heavily in California political races – typically about 80% of votes cast – and those ballots can be counted up to a week after election day, as long as they are postmarked before election day.

California uses a “jungle” primary process, in which the two candidates with the most votes advance to a runoff – regardless of their political party – unless one candidate wins an outright majority. A huge field of 61 candidates fragmented the vote in the race for governor, but Republicans have coalesced around Steve Hilton. Together with Xavier Becerra and Tom Steyer, the three are in a contest that remains too close to call as votes are tallied.

Mail-in ballots tend to favor Democrats, which implies the possibility that Hilton – whom Trump has endorsed – may drop into third place by the time all the ballots are counted.

The last Republican to win the California gubernatorial race was Arnold Schwarzenegger in 2006. Republicans have polled just under 40% in each of the last four contests.

Last month Gavin Newsom sent a letter to elections officials to thank them for their work while warning that a long process invites disingenuous accusations of misconduct.

“We must acknowledge that the longer the vote count takes, the more mis- and dis-information spreads,” wrote the California governor. “That means we must do all that we can to tabulate votes quickly and accurately. Time is of the essence in preventing election lies from taking root.”

On Wednesday evening, election observers echoed those concerns. “Conducting elections with integrity and ensuring that every eligible vote is counted are fundamental to maintaining public confidence in our democracy,” said Mike DuHaime of the Democracy Defense Project, a bipartisan effort to combat election misinformation.

“At the same time, prolonged delays in ballot tabulation, such as those that have become increasingly common in California, can undermine public trust and create unnecessary uncertainty around election outcomes,” DuHaime added. “The longer election results remain unresolved, the greater the opportunity for misinformation and speculation to spread online, eroding confidence in our electoral process. Accuracy must always remain the highest priority, but accuracy and timeliness are not mutually exclusive.”