A Republican sheriff who is running for governor of California recently seized more than 650,000 ballots cast in a 2025 statewide election, prompting criticism from the state’s top election official, who said the sheriff’s concerns about fraud “lack credible evidence.”

The sheriff, Chad Bianco, on Friday said he was investigating allegations by an election activist group that vote tallies did not match the number of ballots received. “This investigation is simple,” he said at a news conference. “Physically count the ballots and compare that result with the total votes reported.” His office confirmed the ballot seizure and investigation on Sunday.

Mr. Bianco’s investigation involves ballots cast in the 2025 special election over Proposition 50, which asked voters whether they wanted to allow Democrats to redraw the state’s congressional districts in response to the nationwide redistricting war Republicans sparked in Texas last year. The outcome was not close: Voters overwhelmingly supported the redistricting effort, with about 7.4 million voting in favor and 4.1 million voting against it.

State and local and election officials have described the investigation and Mr. Bianco’s justification for seizing the ballots as baseless. Democrats and Republicans in the state have also said they believe that politics was a motivating factor in the probe. When asked for comment, Mr. Bianco, the Riverside County Sheriff, criticized the state’s Democratic attorney general.

The moves by Mr. Bianco, a supporter of Mr. Trump, are similar to efforts by the Trump administration and its allies to discredit previous elections. Such efforts could be used as a justification for attempts to change the way future elections are conducted, election experts have said.

The outcome of the probe is unlikely to have any impact on the result of the Proposition 50 vote, given the margin. But it has raised alarm among election officials about future election interference and accusations in both parties that Mr. Bianco is trying to enhance his standing with the party base — and Mr. Trump — by raising an issue that animates the president and many of his backers.

“It looks to me like it’s a politically motivated effort,” said Jon Fleischman, the former executive director of the California Republican Party. He added: “It’s awfully coincidental that he would be taking this high profile and extreme of an action literally two months before he’s facing a statewide election.”

Shirley Weber, the Democratic secretary of state, denounced the investigation and said the claims made by Mr. Bianco were not backed by evidence. Moreover, a sheriff’s department was not properly equipped to conduct such an election review, she said.

“The Riverside County Sheriff’s Office has taken actions based on allegations that lack credible evidence and risk undermining public confidence in our elections” Ms. Weber said in a statement on Friday. She added: “Investigations into election processes must be conducted by those with the appropriate legal authority and subject matter expertise. Similar claims raised in other states by individuals without election administration experience have been thoroughly reviewed and debunked.”

Rob Bonta, the Democratic attorney general in California, issued a stern rebuke of Mr. Bianco’s investigation in a letter earlier this month, saying the office had “serious concerns” about the underlying evidence and that Mr. Bianco was “flagrantly violating my directives.”

Mr. Bonta took particular issue with Mr. Bianco’s plan to use his own staff to audit the ballots.

“Let me be clear: this is unacceptable,” Mr. Bonta wrote. “Your decision to seize ballots and begin counting them based on vague, unsubstantiated allegations about irregularities in the November special election results sets a dangerous precedent and will only sow distrust in our elections.”

In a statement, Mr. Bianco criticized Mr. Bonta and said he viewed the attorney general’s actions as interfering with his election.

“A judge approved the warrant, so Bonta’s opinion means absolutely nothing,” Mr. Bianco said.

The investigation stems from allegations from a group called Riverside Election Integrity Team. The group said its examination of records found about 45,000 more ballots had been counted than were documented as having been received, according to Mr. Bianco. But local election officials quickly dispelled the allegations, noting that they were rooted in a misunderstanding of how ballots are officially counted, according to The Palm Springs Desert Sun.

“County election staff follow detailed procedures established by state and federal law to protect the integrity of the vote and to ensure that every eligible ballot is processed and counted in accordance with those legal requirements,” Riverside County Executive Officer Jeff Van Wagenen said in a statement. He said he would not comment on the sheriff’s investigation or remarks.

Mr. Bianco is locked in a sprawling all-party primary election for governor in California, a heavily Democratic state. The top two candidates, regardless of party, will advance to the general election — and with many Democrats in the field, a vote split that opens the door for Republicans remains possible.

Mr. Bianco has polled near the top of the field. But he has faced criticism from some on the far right, including Laura Loomer, the activist with close ties to Mr. Trump. The investigation into voter fraud, a central issue for Mr. Trump, has been derided by critics of Mr. Bianco as an attempt to build support with Trump and the right-wing base, as he competes with another Republican candidate, Steve Hilton, a former Fox News host.

While Mr. Bianco’s effort is focused on a previous election, Democrats and election experts in the state voiced concern that it could be a preview of potential intrusions into the midterms.

Conservative activists have previously courted sheriffs to join their efforts to investigate past and present elections. Ahead of the 2022 midterms, two right-wing sheriffs’ groups endorsed a plan for local law enforcement officials to play a more active role in elections.

At least three sheriffs involved in the 2022 effort — in Michigan, Kansas and Wisconsin — carried out their own investigations, clashing with election officials who warned that they were overstepping their authority and meddling in an area where they had little expertise.

The upheaval in California is happening as the federal government has taken drastic actions to investigate the 2020 election as part of the president’s crusade to find voter fraud, which is exceptionally rare in the United States. In late January, the Federal Bureau of Investigation raided a warehouse in Fulton County, Ga., seizing roughly 700 boxes of election materials from the 2020 election, including original ballots, envelopes, applications and other materials.

The affidavit supporting the raid in Georgia revealed that the justification for seizing the ballots was based on a host of false and disproved claims about the election. Nearly every issue had already been investigated and dismissed by state officials.

“These ballot seizures are setting a dangerous precedent,” Gowri Ramachandran, director of the elections and government program at the Brennan Center for Justice, wrote on social media. She noted that there are clear avenues for contesting an election, such as recounts and lawsuits.

Laurel Rosenhall and Tim Balk contributed reporting.