Tesla controlled 73% of the battery electric market in California from January to September, the lowest level since 2018, according to new vehicle registration data from the California Energy Commission. Tesla had a 75% share in 2021 and 79% in 2020.

Tesla's sales in California - home to its first factory - accounted for 15% of the automaker's global deliveries last year, according to Reuters' calculation. California is the biggest U.S. state for zero emission vehicles, which also includes hydrogen fuel cell, and plug-in hybrid-electric vehicles, and accounts for 18% of new car sales in California this year.

Tesla still sold more electric vehicles in this key market than during the same period a year ago, but rivals including Hyundai Motor stepped up sales of newer models, giving buyers more choice.

Tesla also topped brand consideration, with 53% of potential electric vehicle purchasers considering Tesla this year, according to a study by California auto consultancy AutoPacific, though that was down from 58% in the previous survey last year.

"We are reaching a saturation point for Tesla market share in California," said Ed Kim, president of AutoPacific. He expected rivals to continue to take away market share, while also expecting Tesla sales would still grow.

Tesla CEO Elon Musk's pursuit of Twitter and embrace of Republicans has sparked concerns about Tesla's brand, especially in liberal states like California.

Guidehouse Insights analyst Sam Abuelsamid said the Twitter saga could have deterred some potential Tesla buyers who had previously supported Musk and his clean energy mission.