WASHINGTON – Twenty-one senators called on the Environmental Protection Agency on Friday to let California implement stricter restrictions on vehicle emissions, which other states could then enact.

The National Highway Traffic Safety Administration said this week that new gas mileage rules would pre-empt state emissions standards, leading to fears that EPA would refuse to give California the waiver it wants to restrict greenhouse gas emissions from vehicles.

Because California began regulating pollution before the federal government did, it has special authority under the Clean Air Act to set its own vehicle pollution standards if it gets a waiver from EPA. Other states can adopt either the federal standards or California’s rules.

California air regulators approved rules in 2004 to cut exhaust from cars and light trucks by 25 percent and from larger trucks and sport utility vehicles by 18 percent starting in 2009. Ten other states have also adopted those standards, which are opposed by automakers, but the rules can’t be implemented until California gets a waiver. EPA has yet to grant one.

“In the face of federal inaction on global warming, California, Maine and the other states have stepped forward to begin reducing the pollution that causes global warming. We are concerned that you may be preparing to argue that the states lack authority to do so,” said a letter to EPA Administrator Stephen Johnson signed by Sens. Dianne Feinstein, D-Calif., Olympia Snowe, R-Maine, and others.

Snowe was among five Republicans who signed the letter, as did independent Sen. James Jeffords of Vermont and 15 Democrats.

“Rather than attempting to thwart such state efforts, the federal government should encourage states to develop innovative solutions to serious public health and environmental problems,” the letter said.

EPA spokesman John Millett said he had no immediate comment, noting that the agency still was reviewing the waiver request.

The National Highway Traffic Safety Administration on Wednesday rolled out tighter gas mileage rules for pickups and sport utility vehicles. The agency argued in its rule that Congress has “expressly pre-empted” states from issuing their own rules on mileage standards, and that any state law limiting carbon dioxide emissions would qualify as such because it would have the direct effect of regulating fuel consumption.

The 10 states that have adopted California’s vehicle emissions rule are Maine, Connecticut, Massachusetts, New Jersey, New York, Oregon, Pennsylvania, Rhode Island, Vermont and Washington.