The brothers are also thought to be the driving financial force behind Americans for Prosperity, the conservative political action committee that spent tens of millions during the election. The group has stayed politically active in recent weeks, rallying support against "fiscal cliff" plans that would increase tax revenues.

In the letter to fellow fundraisers, Charles Koch acknowledged that he found November's election results "disappointing," but expressed optimism about future.

"Our goal of advancing a free and prosperous America is even more difficult than we envisioned, but it is essential that we continue, rather than abandon, this struggle," Koch said.