WASHINGTON — The U.S. Postal Service plans to substantially increase the number of electric-powered vehicles it’s buying to replace its fleet of aging delivery trucks, officials said Wednesday.

The Postal Service anticipates boosting electric vehicles from 20% to 50% in its initial purchase of 50,000 vehicles — with the first of them rolling onto delivery routes next year. It also proposes buying an additional 34,500 commercially available vehicles over two years, officials said.

The proposal, to be posted in the Federal Register on Thursday, came after 16 states, environmental groups and a labor union sued to halt purchases of next-generation delivery vehicles under the initial plan that was skewed toward gas-powered trucks.