A bipartisan group of senators on Friday chided federal regulators for pulling back on the federal ethanol mandate.

In a letter to Environmental Protection Agency (EPA) Administrator Gina McCarthy, the 39 senators said the agency should "take the opportunity to get the program back on track by setting blending targets where Congress intended” when regulators finalize 2017’s standards this year.

Sens. Chuck Grassley (R-Iowa) and Amy Klobuchar (D-Minn.) led the letter; twelve Republicans, mostly from Midwestern states, signed on to the letter, along with Democrats from around the country.

The EPA in May rolled out it is 2017 Renewable Fuel Standard levels in May, proposing to instruct refiners to mix 18.8 billion gallons of biofuels into the country’s gasoline and diesel supply next year.

The level is higher than it's been before, but still well below the levels Congress envisioned for 2017 when it passed the standard.

The oil industry says it is blending as much fuel as it can into the oil supply. It has also sought a waiver from higher ethanol levels, citing a lack of fuel distribution infrastructure across the country.

Biofuel interests, however, have lambasted the standards for being too low.

The EPA defended its new standards this week, with an official telling a House panel, “we think we are doing what we’re supposed to do, which is to look at the information, to talk to everybody to understand the industry as well as we can and do our very best to implement what we understand the intent of Congress to be, which is to have more renewable fuels in our transportation supply.”

Even so, industry groups have pushed the EPA to boost the levels it has proposed for 2017 and beyond. Ethanol supporters in Congress have done the same.

“We urge you to ensure that the final rule promote growth in the U.S. biofuel sector and capture economic opportunity rather than drive investment overseas,” the senators wrote in their Friday letter.