WASHINGTON — Senate Democrats have questioned whether President Donald Trump’s nominee for a top EPA position is violating the law by working at the agency before being confirmed, and they are demanding more details about his duties.

Michael Dourson, Trump’s pick to become the country’s top regulator of toxic chemicals, is serving at the Environmental Protection Agency as a special adviser, even though the Senate has yet to vote on his nomination.

“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,” 10 Democrats wrote in a letter to Dourson, warning that it would be "unlawful" for him to assume the duties of the position to which he's been nominated.

If Dourson is exercising the authority of that Senate-confirmed position, going beyond an advisory role at the agency, he could be at odds with the 1998 Federal Vacancies Reform Act. Senate Democrats have expressed similar concerns about Susan Bodine, Trump's pick to lead EPA enforcement, and some experts raised questions about other Trump nominees working for the administration while awaiting confirmation.

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The EPA denies that there's anything improper about Dourson’s work at the agency, pointing out that many previous administrations have done the same. In his current role as a senior adviser, Dourson is not “performing the duties” of the position he has been nominated for, “which is what occurred in previous administrations,” said EPA spokesperson Jahan Wilcox.

In March, the Supreme Court ruled that the Obama administration had violated the Federal Vacancies Reform Act by having an official serve as acting counsel at the National Labor Relations Board while awaiting Senate confirmation.

The Senate Environment and Public Works Committee is scheduled to vote on whether to send Dourson’s nomination to the full Senate on Wednesday morning. The Republican leadership delayed the vote, originally scheduled for last week, shortly after the news broke about Dourson’s current role at the agency and days before The New York Times published an exposé on another EPA official, Nancy Beck, a former executive for the chemical industry who has collaborated on research with Dourson.

Related: Democrats: Trump EPA Nominee Michael Dourson Is Toxic Chemical Swamp Creature

Senate Democrats and public health advocates have vehemently opposed Dourson’s nomination given his deep ties to the chemical industries that he would be charged with regulating at the EPA.

Spearheaded by Sen. Sheldon Whitehouse of Rhode Island, the Democrats' letter demanded more information from Dourson about his current duties at the EPA. They also asked whether he is still abiding by the ethics agreement that he had signed during the confirmation process. Dourson spent decades conducting industry-funded research, prompting Democrats to question whether he'd recuse himself from issues related to those companies.

The EPA said that it has "briefed Dr. Dourson about his ethics issues," giving him advice that is akin to the agreement that he previously signed, according to EPA senior counsel Justina Fugh.