WASHINGTON — The Trump administration is making a concession on its proposed minimum fuel economy requirement for new vehicles, but environmental groups and a key Democratic senator say it does not go far enough.

Fuel economy standards would increase 1.5 percent per year from 2021 through 2026 under the new proposal. That’s a reversal from the Trump administration’s proposal in 2018, which sought to freeze the standards at 2020 levels.

Environmentalists and Delaware Sen. Tom Carper hardly cheered the move, which doesn’t come close to the 5 percent annual increase the Obama administration had mandated.

The Office of Management and Budget reviews proposed regulations before they are finalized and printed in the Federal Register. The administration hasn’t released the numbers, but they are detailed in Carper’s letter to Paul Ray, a management and budget administrator.

The mileage rollback has been one of the most fiercely contested rollback efforts, prompting legal battles with California and splitting loyalties of top automakers.