Chalk up one more state in California's column.

Colorado has added its name to the list of 12 states that follow California's tighter emissions standards.

Governor John Hickenlooper directed Colorado's Air Quality Control Commission to begin the legal process of bringing the state into the coalition of those that follow California's air quality rules, rather than the federal EPA's.

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Under the Clean Air Act, California is allowed to set stricter emissions requirements for cars than the EPA, and other states are allowed to opt-in to California's standards or to follow looser federal guidelines. They cannot set their own.

As a state historically tied to the oil and mining industries, Colorado's move stands out. It is the only mid-continent state to sign on to California's clean car standards. Like southern California, the populated Colorado Front Range is a region that has had historic problems with smog accumulation. Several Colorado counties including those that contain the Denver metro area are required by federal law to conduct emissions testing.

Hickenlooper's move comes in response to EPA Administrator Scott Pruitt's announcement in May that he would rescind the agency's tightening emissions standards that are tied to rising fuel economy standards. The EPA agreed to the standards in coordination with California and the NHTSA, which has authority over fuel economy standards. Under the new proposal, federal emissions and fuel-economy standards would be capped at 2021 levels through 2026.

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Last month, the EPA also threatened to cancel the waiver that allows California to set its own emissions standards, a right it has had under the Clean Air Act since 1970.

Currently, 13 other states along with Washington, D.C., follow California's emissions standards. Nine of those states also follow the California mandate that requires a certain percentage of cars sold in the state to be zero-emissions electric or cars. That electric car mandate is widely credited for the development of today's generation of electric cars on the market.

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It is unclear yet whether Colorado will also follow California's zero-emissions vehicle mandate. Colorado currently has the largest incentives for electric cars of any state: a $5,000 tax credit alongside as much as $3,000 from one of the state's largest utilities.

Although other states follow California's Zero Emissions Vehicle requirements, it has been a challenge to get automakers to sell electric cars outside of California and Oregon.

Last week, the Alliance of Automobile Manufacturers sent a letter to the governors of nine other states that follow California emissions regulations urging them to do more to promote electric-car sales and installation of public charging infrastructure in their states to promote electric-car sales.

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Under the arcane rules that govern the California zero-emissions vehicle mandate, automakers will no longer be able to count sales of electric cars in California to meet the mandates in these other states starting this year. If it adopts the zero-emissions vehicle mandate, Colorado would join Oregon as the only other state west of the Mississippi whose electric car sales could be pooled with those in California to help automakers comply with the mandate.

That, plus its high incentives for electric cars, could make Colorado one of the more popular states for electric car sales.

Environmental groups were quick to praise the decision.

“With the Trump administration abdicating leadership on cleaning up tailpipe pollution and saving consumers money on gas, states need advanced vehicle standards to ensure their citizens get to drive the cleanest, most affordable cars on the market. This action will help ensure Coloradans still get clean air and cleaner cars,” said Noah Long, senior attorney for the Natural Resources Defense Council.