Scott Pruitt, who had a scandal-ridden spell in charge of the Environmental Protection Agency under Donald Trump, filed on Friday to run for an open US Senate seat in Oklahoma.

Pruitt, 53, a former state senator and Oklahoma attorney general, is seeking the seat being vacated by the longtime Republican senator Jim Inhofe.

Pruitt stepped down as EPA administrator in 2018 amid a wave of ethics scandals, including living in a bargain-priced Capitol Hill condo tied to an energy lobbyist.

He also faced ethics investigations into pricey trips with first-class airline seats and unusual security spending, including a $43,000 soundproof booth for making private phone calls.

He also demanded 24-hour-a-day protection from armed officers, resulting in a swollen 20-member security detail that blew through overtime budgets and racked up expenses of more than $3m.

Like Trump, Pruitt was a staunch advocate for the continued use of coal and other fossil fuels, voiced skepticism about mainstream climate science and was a fierce critic of the Paris climate agreement.

Trump cheered Pruitt’s moves to expand fossil fuel production and roll back regulations opposed by corporate interests. After leaving the agency, Pruitt registered as an energy lobbyist in Indiana.

In a brief interview on Friday, Pruitt said he “led with conviction in Washington DC” and chalked up the criticism against him as resulting from leading an agency that was the “holy grail of the American left”.

“And I made a difference in the face of that,“ Pruitt said. “I think Oklahomans know when the New York Times and CNN and MSNBC and those places are against you, Oklahomans are for you.”

Pruitt will face a crowded primary field seeking to replace Inhofe, 87, who shook up Oklahoma politics with his announcement that he would step down in January, just two years into a six-year term.

Ten Republican hopefuls have filed to run for the seat, including the congressman Markwayne Mullin, former speaker of the Oklahoma house TW Shannon, Inhofe’s longtime chief of staff, Luke Holland, state senator Nathan Dahm and Alex Gray, former chief of staff of the national security council under Trump.

A former congresswoman, Kendra Horn, was the only Democrat to file for the seat by early afternoon on Friday, the final day of the three-day candidate filing period.

Because of Inhofe’s announcement, both of Oklahoma’s US Senate seats are up for grabs this cycle. The current senator James Lankford is seeking another six-year term.

Lankford will face the Tulsa pastor Jackson Lahmeyer in the GOP primary. Four Democrats, a Libertarian and an independent also filed this week to seek the post.

The most competitive US House race is expected to be for the open second congressional seat in eastern Oklahoma, which is being vacated by Mullin. At least 13 candidates, 12 Republicans and an independent, have filed for that post.