Updated at 10:36 a.m. ET

The Trump administration has proposed a rollback of Obama-era fuel efficiency and emissions standards, while simultaneously taking aim at California's unique ability to set more stringent rules.

Under the Obama administration, the Environmental Protection Agency called for the fuel economy standards for new vehicles to ratchet up over time. The increasingly strict standards were designed to combat climate change by reducing greenhouse gas emissions.

On Thursday, the EPA and the National Highway Traffic Safety Administration released a new proposed rule that would instead freeze the standards at their 2020 levels for six years.

The agencies say that increasing fuel efficiency requirements contributes to an increase in the cost of new cars and trucks, which may discourage consumers from buying new vehicles. Because newer vehicles have advanced safety features, the administration argues, increasing fuel economy requirements therefore harms highway safety, as well as having economic effects.

"Cars and trucks are just part of the basic fiber of the American economy and the American experience so we take what we're doing very, very seriously," Bill Wehrum, EPA assistant administrator, told reporters on Thursday.

The EPA has argued that the federal standards imposed by the Obama administration are burdensome on auto manufacturers. However, "major automakers have repeatedly said they do not back freezing the requirements but have called for changes to take into account fuel prices and shifting consumer demand," Reuters says.

The proposed rules also call for eliminating California's power to set its own auto emissions standards, a powerful tool the state has used to try to curb greenhouse gas emissions.

Because of a federal waiver, California has been allowed to set its own tailpipe emissions standards that are more stringent than federal law.

This is a privilege that California alone has, as NPR reported: "Other states can follow California's lead or the federal government's lead but they aren't allowed to strike out on their own."

The proposed rules call this practice a "fundamental and unnecessary complication" to regulations, and proposed replacing them with a less-stringent set of rules applying to all 50 states.

"California will fight this stupidity in every conceivable way possible," California Gov. Jerry Brown said in a statement on Thursday.

California Attorney General Xavier Becerra called the proposed rules "a brazen attack, no matter how it is cloaked, on our nation's Clean Car Standards."

At least 12 other states have adopted California's standards, and together they form a powerful unit. As NPR's Nate Rott reported, "together they account for a third of all car sales in the U.S."

California, 16 other states and the District of Columbia sued the EPA in May in anticipation of the EPA action, NPR has reported: "The states are asking a court to review the EPA's proposed actions, arguing that they violate the Clean Air Act."

California prides itself as a pioneer in pushing for clean air – an approach that was born out of its own severe air pollution issues. In the 1940s, the smog was sometimes so thick in Los Angeles that visibility was just three blocks, according to the California Air Resources Board.

For decades, the state has argued that it needs stricter standards "due to California's unique geography, weather and expanding number of people and vehicles," CARB adds.

California has already reached its emissions reduction target for 2020. As the Los Angeles Times reported, it has "hit its target to reduce greenhouse gas emissions below 1990 levels." Now, it's trying to cut emissions even further, aiming for an additional 40 percent by 2030.

But by far the largest source of California's greenhouse gas emissions are automobiles. And as the Times reported, those numbers have been rising.

California's regulations also require car companies to produce and deliver a specific percentage of vehicles that are zero emissions — a framework that other states have also adopted.

Copyright 2018 NPR. To see more, visit http://www.npr.org/.