148,241 California mail ballots were rejected in the June primary, equal to 1.73% of all mailed votes and up from 108,982 in the 2024 primary.
93,479 of those rejections stemmed from lateness, with voting experts pointing mainly to ballots postmarked after Election Day rather than ballots arriving beyond the state's seven-day acceptance window.
USPS processing changes are under scrutiny because updated policy can tie postmarks to handling at distant processing centers, and California officials had already warned voters earlier this year to mail ballots sooner.
Nearly 44,000 ballots were rejected for signature mismatches and more than 8,300 for missing signatures, while Tulare County posted the state's highest rejection rate at 3.52%.
The spike adds pressure on California's vote-by-mail system and slow count process, even as there is no evidence of widespread mail-voting fraud.