(Reuters) - Six U.S. Senate Democrats on Thursday stepped up efforts to get more clarity from the White House and chief of the Environmental Protection Agency on Carl Icahn’s role as special adviser on regulations to President Donald Trump.  The lawmakers, including Sheldon Whitehouse of Rhode Island and Elizabeth Warren of Massachusetts, in letters to the White House counsel and to EPA Administrator Scott Pruitt, raised concerns about the billionaire investor’s conflicts of interests. They highlighted his involvement with a potential overhaul of the U.S. biofuels program.  Icahn has weighed in on the issue, which could cut millions of dollars in costs for refiners like CVR Energy Inc, in which he owns an 82 percent stake. “We are concerned that his substantial and widespread private-sector investments present perverse incentives for Mr. Icahn in his role as a special adviser to the President,” the letter to White House Counsel Donald F. McGahn II said.  Previous letters seeking details on Icahn’s role have gone unanswered amid signs of Icahn’s involvement on the issue, the senators said. Efforts to reach Icahn or his representatives were not immediately successful. Oil refiners like CVR Energy and Valero Energy Corp have pressed for a change to the Renewable Fuel Standard to meet annual biofuel standards from further downstream to companies  like fuel retailers.  The Renewable Fuel Standard requires oil refiners blend biofuels with gasoline and diesel, or buy credits from firms that have. Refiners have asked the government to shift those requirements from them further downstream to fuel retailers. The RFS has become a battleground between entrenched farm and oil interests.  A biofuels lobbyist earlier this month said Icahn told him an executive order on the issue was forthcoming, a claim that both the adviser and the White House have denied. The events of the last two weeks have underscored worries over Icahn’s conflicts of interest, the senators said.  Icahn “has taken the first opportunity to leverage his newfound political power for his own personal gain,” the letter stated, as the senators pressed for more specifics on his conversations with the Trump and Pruitt on the matter. Icahn has weighed in heavily on this issue as costs for oil refiners hit record levels in 2016. In August, he pressed the EPA under former President Barack Obama to overhaul the market for renewable fuels credits. Prices of those credits tumbled over a third on the day the White House denied any plans to issue an executive order on the matter. Shares of CVR stock jumped as much as 8 percent that day. 