Sen. Dianne Feinstein Dianne Emiel FeinsteinTrump plans to pick Amy Coney Barrett to replace Ginsburg on court Trump faces tricky choice on Supreme Court pick The Hill's Morning Report - Sponsored by Facebook - Trump previews SCOTUS nominee as 'totally brilliant' MORE (D-Calif.) is asking a government watchdog to probe a threat from the Trump administration over her state's highway funding, saying she's worried “California is being unfairly targeted.”

In a letter sent to EPA Deputy Inspector General Charles Sheehan on Friday, Feinstein requested an investigation into potential “inappropriate political interference” resulting from an EPA letter to California officials threatening to withhold federal highway funding over the state’s poor air quality .

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The letter sent Tuesday from EPA Administrator Andrew Wheeler Andrew WheelerOvernight Energy: Trump officials finalize plan to open up protected areas of Tongass to logging | Feds say offshore testing for oil can proceed despite drilling moratorium | Dems question EPA's postponement of inequality training Democrats question EPA postponement of environmental inequality training OVERNIGHT ENERGY: California seeks to sell only electric cars by 2035 | EPA threatens to close New York City office after Trump threats to 'anarchist' cities | House energy package sparks criticism from left and right MORE to the California Air and Resources Board listed a number of backlogs for implementing state air pollution plans the EPA argues are out of compliance with federal standards.

Feinstein requested the IG office look into whether the claims of negligence truly fall with California officials.

“I ask that you investigate whether these reports are, in fact, backlogged as a result of inaction on the part of California jurisdictions,” she wrote. “For example, the letter cites a 1997 state implementation plan from Coachella Valley regarding key ozone National Ambient Air Quality Standards. However, the report was previously submitted to EPA and is awaiting EPA approval, not additional county action.”

Feinstein added: “I am concerned that California is being unfairly targeted, and that this issue of backlogged state implementation plans is nothing more than a pretext to attack California, rather than a good-faith effort to help improve California’s air quality.”

Wheeler’s letter came a week after the agency moved to revoke the state’s ability to regulate tailpipe emissions at a higher level than the federal government. Vehicle emissions are one of the largest sources of air pollution in California. California and 23 other states sued the Trump administration last Friday over the rule.

“It would certainly seem counter-intuitive for EPA to sanction California for poor air quality while at the same time attempting to remove one of California’s biggest tools to improve its air quality,” said Feinstein.

An EPA spokesperson denied the agency's letter was political in nature.

"EPA’s actions are not political. We will continue to work toward fulfilling the mission of the Agency," the spokesperson said in a statement.