Nearly 150,000 California voters had their mail-in ballots rejected in the state's June primary — a record high driven largely by ballots arriving too late to be counted, according to data from the secretary of state's office.

The 148,241 rejected ballots represent a rejection rate of 1.73% of all mail ballots cast. That marks a sharp increase from the 2024 primary, when 108,982 ballots were rejected at a 1.56% rate, and the 2022 primary, which saw 105,818 ballots voided at a similar rate to 2024. The numbers take on added weight given that California, the nation's most populous state with over 22 million registered voters, automatically mails a ballot to every registered voter under a law made permanent in 2021 — meaning even a sub-2% rejection rate can affect a large number of people.

Late Postmarks Identified as Primary Cause

Lateness accounted for 93,479 rejected ballots in June — by far the largest disqualification category. That figure rose from approximately 70,000 in the 2022 primary and roughly 76,000 in the 2024 primary, even though turnout in those earlier elections was smaller.

Voting experts attribute the problem primarily to postmarks applied after Election Day, rendering ballots ineligible. The suspected cause involves U.S. Postal Service operational changes that may affect when postmarks are stamped.

"Ballots rejected due to lateness are caused primarily by being postmarked too late to count, not because they arrived too late to count," Kim Alexander, president of the nonpartisan California Voter Foundation, wrote in an email.

Alexander noted that the spike in lateness-related rejections — potentially linked to postal procedures and a late surge of ballots in June, when many voters delayed decisions in a wide-open gubernatorial race — is a major concern.

Mindy Romero, director of the Center for Inclusive Democracy, a nonpartisan research group, said that while rejection rates have fluctuated over time, the growing number of ballots set aside for lateness "does really demand the question why this is happening."

Postal Service Changes Under Scrutiny

California allows mail-in ballots to arrive at election offices up to seven days after Election Day, provided they are postmarked by Election Day. Despite this generous window, late-arriving ballots remain the top rejection driver.

In January, a group of mostly Democratic U.S. senators sent a letter to the U.S. Postal Service expressing alarm that mail-processing changes might affect postmark dates during an election year that will determine control of Congress.

Updated Postal Service policy states that postmarks may indicate the day mail was handled at a processing center rather than the first day the agency received it. Recent consolidations within the Postal Service mean those processing centers are increasingly likely to be located further from certain communities, potentially delaying postmarks further.

On Thursday, two Wisconsin Democrats — U.S. Sen. Tammy Baldwin and Gov. Tony Evers — called on the Postal Service to address delays that caused hundreds of ballots to arrive after Election Day in their state in April. In June, a federal judge halted President Donald Trump's executive order seeking to create a federal voter list and restrict mail ballot access.

Earlier this year, California election officials urged voters to return mail ballots early, warning that Postal Service changes could slow delivery. With the November general election ahead — when turnout typically far exceeds primary levels and control of Congress is at stake — the postmark issue could affect an even larger pool of ballots unless postal procedures or voter behavior changes.

County-Level Rejection Rates and Other Disqualification Reasons

The highest rejection rate in June — 3.52% — was recorded in Tulare County, located in the agriculture-heavy Central Valley. Alpine County, south of Lake Tahoe, and Merced County, also in the Central Valley, followed with rates of 3.36% each.

Beyond lateness, nearly 44,000 ballots were rejected because the voter's signature did not match the one on file with county officials. Over 8,300 ballots lacked a signature on the return envelope. In 743 cases, the voter had already cast a ballot, voiding the mail submission. Additional reasons included missing ballots in return envelopes and multiple ballots returned in a single envelope.

The data did not break down uncounted ballots by party registration.

California's Vote-Counting Reputation and Political Fallout

While large states such as Florida and Texas complete vote counts quickly, California is known for prolonged counting processes that can extend for weeks. After the June primary, President Donald Trump cited California's slow count to renew criticism of the state's elections, and the Department of Justice launched an investigation into Los Angeles County's elections.

There is no evidence of widespread problems with mail voting, which has grown in popularity among both Democrats and Republicans. A 2025 report by the Brookings Institution found mail voting fraud cases to be minuscule — approximately four cases per 10 million mail ballots.

This story was originally featured on Fortune.com.