An extension of a consumer tax credit for electric vehicle purchases sought by a bipartisan contingent of lawmakers could cost as much as $15.7 billion, according to an analysis prepared by opponents of the effort.

The analysis of legislation by Senator Debbie Stabenow, a Michigan Democrat, to grant automakers a $7,000 tax credit for an additional 400,000 vehicles was prepared by American Fuel & Petrochemical Manufacturers, a Washington trade group.

The current $7,500 per-vehicle incentive for consumers, credited with helping establish the nascent market for electric cars, phases down once a manufacturer sells 200,000 of the vehicles.

Stabenow’s bill would...