The conservative patrons Charles and David Koch haven't shown up in rankings of the country’s top political givers because the brothers largely back tax-exempt groups that do not reveal their donors -- but big checks they gave to a new super PAC in July now officially place them on the list of this year's most generous campaign donors.

The Kochs each gave $2 million to Freedom Partners Action Fund through trusts in their names, part of a $15 million haul the group brought in between July and September.

The super PAC was formed this summer as a way to sharpen the impact of a political network supported by the Kochs and hundreds of conservative donors. The group can use all of its funds on direct political activity, a latitude it has embraced as it has pounded congressional Democratic candidates around the country.

While the Kochs are some of the biggest donors, Freedom Partners Action Fund reported receiving hundreds of contributions from individuals and corporations in the last three months. The largest donation came from Renaissance Technologies hedge fund executive Robert Mercer, who gave $2.5 million, as Politico first reported.

Wisconsin roofing magnate Diane Hendricks, Arkansas poultry executive Ronald Cameron and Nebraska trucking entrepreneur Clarence Werner each ponied up $1 million. Variety Stores, the discount retail company owned by former North Carolina budget director Art Pope, gave $400,000.

Pope said that the donation was made after he left his state post, and was for general use by the super PAC.

"It was not designated for use in North Carolina or any other geographic area or any particular activity or purpose," he wrote in an emailed statement.