Democratic senators on Tuesday demanded information from the Environmental Protection Agency about how an unconfirmed nominee to lead the toxic chemicals office has already been working at the agency.

Michael Dourson is one of the key EPA officials who face confirmation votes Wednesday before the Senate Environment and Public Works Committee.

The EPA confirmed this month that Dourson is working as a top adviser to Administrator Scott Pruitt.

In previous administrations, including during former President Barack Obama's tenure, nominees have worked in advisory roles at federal agencies as they awaited confirmation. But Dourson's nomination is particularly contentious and his confirmation by the Senate is not promised.

A coalition of 10 senators, led by Democrats Tom Carper of Delaware, Sheldon Whitehouse of Rhode Island, Kamala Harris of California, and Edward Markey of Massachusetts, and independent Bernie Sanders of Vermont, say in a letter to the EPA that Dourson's advisory role at the agency may violate the Federal Vacancies Reform Act.

The obscure federal law prohibits most people who have been nominated to fill a vacant government position from doing that office's duties in an acting capacity.

"Your appointment creates the appearance, and perhaps the effect, of circumventing the Senate's constitutional advice and consent responsibility for the position to which you have been nominated," the senators wrote. "Your improper involvement in EPA decisions could provide grounds for subjects of EPA regulations and oversight to challenge the legal validity of those decisions in court."

The senators asked the EPA to provide a description of Dourson's appointment and a job title for him in his acting capacity.

They ask whether he is overseeing EPA employees and if Pruitt has delegated any of his authorities to him. They request information on the type of email account he is using and whether he will make his schedule public. And they want to know if he has stopped his consulting work for chemical and agriculture companies.

Dourson, a toxicologist and University of Cincinnati professor, faced a tough confirmation hearing this month.

He has been criticized for his ties to the chemical industry, which he would be expected to regulate. He founded a consulting group that represented companies that produced chemicals now under EPA review for their public health risks.

Carper at the hearing presented a list of chemicals Dourson has recommended for lower safety standards than the norm proposed by regulators, often after he was hired by companies.

"It is regrettably difficult to look at your record and conclude you can be an impartial regulator," Carper said.

Dourson would not promise to recuse himself from evaluating chemicals he studied in a private capacity.

The Environment and Public Works Committee last week delayed Dourson's nomination vote, with other EPA nominees, after some Republicans suggested they may not approve them as retaliation for the Trump administration's plan to weaken the EPA's biofuel mandate.

Those concerns from Midwestern Republicans seem to be mollified for now, because Pruitt assured GOP senators later in the week that he would keep intact the Renewable Fuel Standard, which requires a certain amount of biofuels to be blended into the nation's fuel supply.

EPA nominees scheduled for consideration Wednesday include Dourson; Bill Wehrum, tabbed to run the agency's air office; Matthew Leopold, Trump's nominee to be assistant administrator for the Office of General Counsel; and David Ross, chosen to be assistant administrator for the Office of Water.

Republicans hold an 11-10 majority over Democrats on the committee, so just one GOP defection would defeat any one of their nominations.