With 134 billion gallons of gasoline demand forecast for 2014, the 14.4 billion quota for corn ethanol would represent more than 10 percent of gasoline consumption.
Oil refiners have been reluctant to cross that 10 percent threshold - nicknamed the "blend wall" - due to fears over engine damage that higher ethanol blends can cause in many cars on the road today.
Manufacturers of motorcycles, boats and small-engine equipment like chainsaws and lawn mowers are also concerned about damage from the higher ethanol blends, and have sided with refiners in lobbying against the mandates.
Refiners have also raised concerns about the lack of infrastructure - such as pumps at gasoline stations - available to sell gasoline blends higher than 10 percent without risking misfueling.
To address those concerns, the EPA proposed reducing the 2014 corn ethanol blending quota to about 13 billion gallons. That's down 800 million gallons from this year and about 1.4 billion less than the 14.4 billion required by the 2007 law.
Iowa Senator Chuck Grassley, who also attended the EPA meeting, said in a statement that "the blend wall is a result of Big Oil's obstruction to higher ethanol blends, and EPA's proposal rewards its obstruction."
The meeting was attended by 10 Democratic Senators and six Republicans, according to a list released by Harkin. Several big corn-producing states were represented, including Iowa, Indiana, Minnesota, North Dakota, South Dakota and Nebraska, which each had two Senators in attendance.
Michigan Senator Debbie Stabenow, who Chairs the Senate Agriculture Committee, was also in attendance, along with Dick Durbin of Illinois, the Senate's second highest-ranking official.
It is unclear whether the lawmakers will be able to sway the EPA. But Harkin made clear that the political pressure is bound to continue.
"We'll see what happens, but I hope they get the message that we're not going to stand still for this kind of a rule that takes us in the wrong direction," he said.