Automakers have made plenty of promises about electric vehicles. General Motors, Ford, and Volvo—some of the more ambitious—have pledged to sell only zero-emission cars by at least 2035. That’s quite a commitment, as only 14 percent of new cars sold globally last year were electric, with the share in the US being half that.

But a new proposal released by the US Environmental Protection Agency today threatens to hold automakers to their electric big talk—and to up the ante. The agency suggested tighter emissions standards that it calculates…