Midwestern Republicans, meanwhile, pressed Mrs. White on her past criticism of the renewable fuel standard, which requires refiners to blend corn-based ethanol with gasoline. White House officials last month directed the E.P.A. to halt efforts to weaken a federal biofuel mandate after pressure from Iowa politicians and others who threatened to hold up agency nominees.

Image Andrew R. Wheeler is the nominee to be deputy administrator of the Environmental Protection Agency. Credit... Faegre Baker Daniels Consulting

Mrs. White said that her position had recently changed because she had read new data and that she now supported the fuel standard.

If the two are confirmed, it will go a small way toward filling the still largely empty ranks of senior energy and environmental posts, which are slowing the Trump administration’s ability to achieve its agenda. According to the Partnership for Public Service, a nonpartisan and nonprofit organization that tracks federal nominations, 28 key positions at the E.P.A., the Energy Department and the Interior Department that require Senate confirmation lack nominees.

That includes critical positions like the under secretary for nuclear energy at the Energy Department and the director of the United States Geological Survey, who essentially serves as the Interior Department’s chief scientist. At the E.P.A., only the administrator, Scott Pruitt, has been confirmed by the Senate. Five others are awaiting confirmation, including William L. Wehrum to lead the agency’s air office and Michael L. Dourson to head the chemical safety office. Lawmakers acknowledged that Mrs. White and Mr. Wheeler may not have a confirmation vote until the end of the year, and six other E.P.A. positions are still without a nominee.

Even Trump administration supporters who have applauded the E.P.A.’s moves to undo Obama-era climate change rules said they were frustrated.

“You can talk the talk, but you need people to actually walk the walk in rolling back these agendas,” said Nicolas Loris, a research fellow in energy and environmental policy at the Heritage Foundation, a conservative research group. “You need people and staff in place to get these through the process.”