Overall, sales of hybrids and cars powered solely by batteries in China were flat in December compared with November, the association said.

There have been other signs that Tesla was suffering from softening demand for its products in China. The company has cut prices for the Model Y and Model 3, the two vehicles it makes in Shanghai. The company’s website says vehicles can be delivered in as little as a week — last year, waits stretched to months.

Analysts say Tesla could be vulnerable to political tensions between the United States and China. Elon Musk, the company’s chief executive, has courted the Chinese government, suggesting, for example, that Taiwan become a special administrative zone of China as a way of ceding more control to Beijing. Taiwan refuses to relinquish any control to China or compromise its independence, and Mr. Musk’s remark angered leaders in Taipei.

Another concern for Tesla: BYD and other Chinese automakers have begun selling cars in Europe. Their expansion could cost Tesla, Volkswagen and other automakers sales and market share when many European car buyers are replacing combustion engine vehicles for battery-powered cars.

“The major worry now overhead for Tesla is that the demand story, especially out of China, is showing heavy cracks in the armor at a time that E.V. competition is steadily increasing,” Daniel Ives, an analyst at Wedbush Securities, said in a note to clients on Wednesday.