six u.s. senate democrats thursday step efforts get clarity white house chief environmental protection agency carl icahn role special adviser regulations president donald trump lawmakers include sheldon whitehouse rhode island elizabeth warren massachusetts letter white house counsel epa administrator scott pruitt raise concern billionaire investor conflict interest highlight involvement potential overhaul u.s. biofuels program icahn weigh issue could cut millions dollars cost refiners like cvr energy inc own 82 percent stake concern substantial widespread private-sector investments present perverse incentives mr. icahn role special adviser president letter white house counsel donald f. mcgahn ii say previous letter seek detail icahn role go unanswered amid sign icahn involvement issue senators say efforts reach icahn representatives immediately successful oil refiners like cvr energy valero energy corp press change renewable fuel standard meet annual biofuel standards downstream company like fuel retailers renewable fuel standard require oil refiners blend biofuels gasoline diesel buy credit firm refiners ask government shift requirements downstream fuel retailers rfs become battleground entrench farm oil interest biofuels lobbyist early month say icahn tell executive order issue forthcoming claim adviser white house deny events last two weeks underscore worry icahn conflict interest senators say icahn take first opportunity leverage newfound political power personal gain letter state senators press specifics conversations trump pruitt matter icahn weigh heavily issue cost oil refiners hit record level 2016. august press epa former president barack obama overhaul market renewable fuel credit price credit tumble third day white house deny plan issue executive order matter share cvr stock jump much percent day senators press trump detail icahn 's special adviser role