Democratic Sen. Tammy Duckworth placed a hold on two of President Trump's EPA nominees Wednesday, in an effort to prevent a vote on the Senate floor.

The Illinois lawmaker, a member of the Senate Environmental and Public Works Committee, is aiming to block the nominations of William Wehrum, who is bidding to become EPA's top air official, and Michael Dourson, tabbed to run the agency's chemical office.

Democrats have accused the duo of being too closely tied to industry to be effective and faithful enforcers of environmental laws.

Duckworth cited Wehrum's unsatisfactory support for the Renewable Fuel Standard as her reason for opposing his nomination.

"In the ten months that Donald Trump has been president, his administration has launched unprecedented attacks on the Renewable Fuel Standard – attacks that fly in the face of promises Trump made as a candidate to our nation's farmers that he would champion the RFS program if elected," Duckworth said in a statement. "Mr. Wehrum's history of attacking the biofuels industry and his refusal to recuse himself from RFS-related issues despite his well-documented conflicts of interest should alarm all of my colleagues."

The Environment and Public Works Committee on Wednesday was scheduled to consider the nominations of key EPA officials, including Wehrum and Dourson, but it postponed its hearing Tuesday night.

The decision came as some Republicans threatened to vote against EPA nominees because of the Trump administration's proposal to weaken the Renewable Fuel Standard, which requires a certain amount of biofuels to be blended into the nation's fuel supply.

Republicans hold an 11-10 majority over Democrats on the committee, so just one GOP defection would defeat the nominations of Wehrum and Dourson before they receive consideration by the full Senate.

Democrats are uniformly opposed to both nominations.

Wehrum, an energy industry lawyer and former EPA official, would oversee a portfolio dealing with climate change regulations.

The post is widely considered the EPA's second most important job, and Democrats say Wehrum's industry ties would complicate his ability to re-evaluate carbon emissions regulations that EPA Administrator Scott Pruitt has tapped for review, such as the Clean Power Plan.

In his private law practice, Wehrum's clients have included the American Petroleum Institute, American Fuel and Petrochemical Manufacturers, the American Chemistry Council and the National Association of Manufacturers.

Concerns over his industry connections led to Wehrum being rejected by Congress to serve in the same position in the George W. Bush administration.

Dourson has faced scrutiny for his ties to the chemical industry, which he would be expected to regulate. The New York Times reported Dourson founded a consulting group that represented companies that produced chemicals now under EPA review for their public health risks.

Consulting for chemical companies, he has recommended lower safety standards for chemicals than the norm proposed by regulators including the EPA, the Centers for Disease Control and Prevention, and other agencies.

"My constituents needed experts such as yourself to stand up for them and their health rather than running cover for polluters who are wealthy and willing to poison our children as they put profits first," Duckworth told Dourson at his confirmation hearing earlier this month. "Manipulating science to achieve a pre-determined outcome is not what the EPA's Office of Chemical Safety and Pollution Prevention should be about."