Tesla's electric-vehicle registrations in California fell about 12% last year, according to industry data, indicating mounting challenges for the automaker in the key U.S. market.

For years, Tesla has been a leader in the electric vehicle market. The company’s products have pushed envelopes, raised the bar, and generated unprecedented buzz – along with booming sales. However, the days of Tesla’s total dominance in the EV world may be drawing to a close.