Carl Icahn

Sen. Elizabeth Warren and other Democratic lawmakers stepped up their campaign against Carl Icahn on Tuesday, charging that the White House's designated agency ethics official previously provided paid legal services to the billionaire corporate-raider turned Trump administration adviser.

The Massachusetts senator and other Democrats sent a letter to the Office of Government Ethics Director Walter Shaub urging him to provide information about ethics rules and how they might apply to President Trump's deputy White House counsel and designated agency ethics official, Stefan Passantino.

According to the Democrats, Passantino had previously provided paid legal services to Icahn, who is a senior adviser to Trump. Passantino also provided paid legal services to Secretary of Health and Human Services Tom Price and Secretary of Housing and Urban Development Ben Carson, they added.

As a deputy White House counsel and the designated agency ethics official, Passantino is responsible for implementing and overseeing ethics requirements that apply to Icahn and other top government officials.

Specifically, the letter pointed to Passantino's employment at Atlanta-based law firm Dentons US LLP until 2015. Prior to that Passantino was a partner at McKenna, Long & Aldridge LLP between 2003 and 2015. (Dentons merged with McKenna Long & Aldridge LLP in 2015). The letter noted that Passantino's ethics form disclosed that he earned "compensation exceeding $5,000 in a year" from Icahn Capital after providing the activist company with "legal services."

An Icahn official didn't return a request for comment.

The letter is the latest escalation in a campaign launched by Democrats against Icahn over his role in the Trump administration. It comes after Warren and a group of Democratic lawmakers last week demanded a probe into Icahn's activities into the biofuels market, focusing on possible insider trading. 

The Democrats argued that Icahn, as an unpaid adviser to Trump, recommended personnel and policies that they assert caused the price of renewable fuel credits to drop. They argued that the investments resulted in a $50 million turnaround from Icahn's initial investment.

At the center of their concerns is Icahn's majority control of CVR Energy (CVI - Get Report) , an oil refiner that has a significant interest in the renewable fuel credit market. 

In December, Trump tapped Icahn as a special adviser for overhauling federal regulations. At the time, Trump said, "Carl was with me from the beginning and with his being one of the world's great businessmen, that was something I truly appreciated." Trump added that Icahn was not only a business man but a "brilliant negotiator" and someone who "is innately able to predict the future especially having to do with finances and economies."