A coalition of Senate Democrats is pressuring Environmental Protection Agency Administrator Scott Pruitt to recuse himself from litigation that is key to the successful repeal of former President Barack Obama's climate regulations.

"We all continue to believe that you should have committed to recuse yourself extensively from a number of matters during your confirmation process," the Democrats said in a letter sent to Pruitt Friday, but was released Monday. They want a detailed account of which litigation he has recused himself from, especially when it comes to climate rules. They want the answer by April 21.

As the former attorney general of Oklahoma, Pruitt sued the EPA over the Clean Power Plan, the centerpiece of President Obama's climate change agenda. Yet, as the lawmakers pointed out, it is not clear if Pruitt has formally recused himself from the litigation. Recusal would have been required to enact President Trump's executive order targeting the Clean Power Plan, according to the letter.

Under the executive order, Pruitt asked the D.C. Circuit Court of Appeals to hold the lawsuit he helped initiate in abeyance until the EPA finishes conducting its review of the climate regulations.

That action "should have triggered the more limited recusal policy to which you have already and repeatedly agreed," the Democrats said in their letter. "We believe you are required to have secured authorization from the [EPA's] Designated Agency Ethics Officer to participate in these matters.

"We seek your written confirmation that such authorization has been granted, or that you will recuse yourself from these matters going forward," the letter stated.

The letter was led by senior Democratic Sens. Maria Cantwell of Washington, who is ranking member of the Energy and Natural Resources Committee, and Tom Carper of Delaware, who is ranking member on the Environment and Public Works Committee, which has direct oversight of EPA.

Carper led the committee's Democrats to boycott Pruitt's confirmation vote in committee this year. Republicans suspended normal procedural rules to hold a vote without one Democrat present. Carper also tried to stall the confirmation vote on the Senate floor by asking the leadership to wait until the Oklahoma Attorney General's office complied with a court order to release thousands of Pruitt's emails.