WASHINGTON — Scott Pruitt, the administrator of the Environmental Protection Agency, has removed himself from some of the most contentious cases facing it, including challenges to the Obama administration’s Clean Power Plan and a controversial rule related to the Clean Water Act.

Mr. Pruitt’s decision to recuse himself, announced in a memorandum dated May 4, involves legal cases he had been a part of during his prior tenure as Oklahoma’s attorney general. In that role, Mr. Pruitt had been one of the most active state officials in bringing lawsuits against the agency he now leads, challenging federal environmental rules. His recusal had been demanded by Democratic lawmakers.

The memo was obtained by E&E News, a site that focuses on environmental and energy reporting.

During his confirmation hearing in January, Mr. Pruitt came under pressure from Democratic senators, who urged him to recuse himself from any cases he had filed against the agency while serving as the attorney general. “What the American people are expecting here is that the E.P.A. doesn’t turn into every polluters’ ally,” Senator Edward J. Markey of Massachusetts told him then. “The only way to do that is for you to recuse yourself from every case you brought.”

During the hearing, Mr. Pruitt said he would not commit to removing himself from all the cases he had brought, though some might be subject to a one-year period of recusal.