“Your positions are so far out of the mainstream, they are not just outliers, they are outrageous,” said Senator Edward J. Markey, Democrat of Massachusetts. “You have a fringe voice that denies science, economics and reality.”
Asked if she stood by her previous statements, Mrs. White replied, “It’s likely that CO2 has some influence on the climate,” but added that carbon dioxide did not have the characteristics of a pollutant that directly affects human health. “It’s a plant nutrient,” she said.
The Trump administration last week issued a comprehensive report on climate science that said that the planet was definitely growing warmer and that human activities were the predominant cause. Despite those unambiguous findings, administration officials and nominees continue to question the validity of climate change science.
Mr. Wheeler, a former aide to Senator James M. Inhofe, an Oklahoma Republican, faced lighter questioning. He declined to pledge to recuse himself from working on lawsuits that Murray Energy has filed against the E.P.A., saying he would abide by the guidance of the agency’s ethics advisers.
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.
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.”
Senator John Barrasso, the Wyoming Republican who is chairman of the Senate Environment and Public Works Committee, blamed Democrats for holding up the 11 environmental candidates the panel has approved and some 100 Trump administration nominees across agencies. The Senate, for example, has yet to confirm the administrator of NASA.
“All year long Democrats have been putting up roadblocks to President Trump’s nominees,” he said. “It’s time to end this pointless spectacle.”
Mr. Trump has indicated many of the vacancies may be permanent. “I’m generally not going to make a lot of the appointments that would normally be — because you don’t need them,” he told Forbes. “I mean, you look at some of these agencies, how massive they are, and it’s totally unnecessary. They have hundreds of thousands of people.”
Max Stier, president of the Partnership for Public Service, agreed that in some cases confirmations for Mr. Trump’s nominees were taking longer than for other recent presidents. And, he said, Mr. Trump may be correct when he says the more than 4,000 appointments every new president must make are too many and largely unnecessary. Still, he said, with only about 440 people nominated to the 1,200 jobs requiring Senate confirmation, “the fault, bluntly, is predominantly with the executive branch.”
“Nobody has done this quickly or well,” Mr. Stier said. “President Trump is just doing it worse than anyone else so far.”