Iowa’s Sen. Charles Grassley and Sen. Joni Ernst said last week they would consider changes to the Renewal Fuel Standard (RFS) requested by lawmakers from petroleum states. But both stressed that they wouldn’t support any chan­­ges that could hurt de­­mand for biofuels or damage the integrity of the RFS.

The Iowa senators’ statements on potential changes to the RFS came after a meeting of congressional staffs from farm and petroleum states to seek a compromise that would address petroleum refiners’ concerns about the RFS. President Donald Trump, at a White House...