(NewsNation) —California and 15 other states that want the U.S. Postal Service to electrify its mail delivery vehicles are suing to halt purchases of thousands of gas-powered trucks as the agency modernizes its delivery fleet.

Three lawsuits, filed Thursday by the states and environmental groups in New York and California, ask judges to order a more thorough environmental review before the Postal Service moves forward with the next-generation delivery vehicle program.

At issue is the relative inefficiency of the current fleet, in which the trucks only average 8.4 mpg.

The new gasoline-powered vehicles would get 14.7 miles per gallon.

Plaintiffs contend that purchases of fossil fuel-powered delivery vehicles will cause environmental harm for decades to come. The lawsuits could further delay the Postal Service’s efforts to replace the ubiquitous delivery trucks that went into service between 1987 and 1994.

“Louis DeJoy’s gas-guzzling fleet guarantees decades of pollution with every postcard and package,” said Scott Hochberg, an attorney with the Center for Biological Diversity, referring to the postmaster general.

Attorneys general from 16 states — 14 of which have Democratic governors — sued in San Francisco. A separate lawsuit by the Center for Biological Diversity, Earthjustice, CleanAirNow KC and Sierra Club was filed in the same venue. Another was filed by the Natural Resources Defense Council and United Auto Workers in New York.

All three of them target the environmental review underpinning the Postal Service’s planned purchase of up to 165,000 next-generation delivery vehicles over the next decade. a plan that will cost $6 billion.

California Attorney General Rob Bonta said it’s key to stop the process before it’s too late.

“Once this purchase goes through, we’ll be stuck with more than 100,000 new gas-guzzling vehicles on neighborhood streets, serving homes across our state and across the country, for the next 30 years. There won’t be a reset button,” he said.

The Postal Service defended the process it followed under DeJoy, a wealthy former logistics executive and Republican donor who was appointed by a board of governors controlled by then-President Donald Trump.

“The Postal Service conducted a robust and thorough review and fully complied with all of our obligations under (the National Environmental Policy Act),” spokesperson Kim Frum said Thursday in an email.

The Postal Service contract calls for 10% of the new vehicles to be electric, but the Postal Service contends more electric vehicles can be purchased based on financial outlook and strategic considerations.

The percentage of battery-electric vehicles was doubled — to 20% — in the initial $2.98 billion order for 50,000 vehicles.

Environmental advocates contend the Postal Service’s environmental review was inadequate and flawed, and that the contract represented a missed opportunity to electrify the fleet and reduce emissions.

The review process “was so rickety and riddled with error that it failed to meet the basic standards of the National Environmental Policy Act,” said Adrian Martinez, senior attorney on Earthjustice’s Right to Zero campaign.

New York Attorney General Letitia James said the Postal Service used “fatally flawed decision-making,” leading to an outcome that was “fiscally and environmentally irresponsible.” New York is among the plaintiffs.

If the parties can’t agree on a settlement, the lawsuit could drag on for months, possibly into next year, if there are appeals, said University of Richmond School of Law professor Carl Tobias.

All told, the Postal Service’s fleet includes 190,000 local delivery vehicles. More than 141,000 of those are the old models that lack safety features such as air bags, anti-lock brakes and backup cameras.

The new vehicles are taller to make it easier for postal carriers to grab packages and parcels that make up a greater share of volume. They also have improved ergonomics and climate control.

The states that sued are California, Connecticut, Delaware, Illinois, Maine, Maryland, Michigan, New Jersey, New Mexico, New York, North Carolina, Oregon, Pennsylvania, Rhode Island, Vermont and Washington.

The Bay Area Air Quality Management District in California, District of Columbia and city of New York joined that lawsuit, as well.