Normally a 50 percent increase in sales is considered very good. But when the number of electric vehicles sold in the United States grew that much during the third quarter from a year earlier, it was a disappointment.

Carmakers and analysts had expected more. Instead of celebrating, auto executives worried that demand for electric vehicles was slackening, raising questions about their plans to invest tens of billions of dollars to develop new models and build factories.

In recent weeks, General Motors, Ford Motor and Tesla cited slower sales and signs that the economy was weakening in announcing that they would delay that spending. That was a blow to the Biden administration’s plan to fight climate change by promoting zero-emission vehicles, and it cast doubt on whether generous federal tax credits for electric car buyers were working as well as policymakers had hoped.

“Our commitment to an all-E.V. future is as strong as ever,” Mary T. Barra, the chief executive of G.M., told analysts on a conference call last month. But, she added, the market is turning out to be “a bit bumpy.” As a result, G.M. is waiting several months to begin selling some new electric models, including a battery-powered incarnation of the Chevrolet Equinox sport utility vehicle.

Ford and G.M. have staked billions to retool factories and build new ones to produce electric vehicles, batteries and other components. If the carmakers have miscalculated, the consequences could be severe. (Stellantis, the parent company of Chrysler, Jeep and Ram, has not yet begun selling any all-electric vehicles in the United States.)

Sales of electric vehicles in China and Europe are also growing more slowly than they were a few months ago.