David Koch, who with his brother Charles has built a powerful conservative political network, is stepping down from his familys conglomerate and his leadership of Americans for Prosperity, the networks signature political organization.

The 78-year-old is retiring due to declining health, his brother wrote to Koch Industries employees Tuesday. In October 2016, David Koch announced to employees that his health was declining and he had been hospitalized over the summer.

Unfortunately, these issues have not been resolved and his health has continued to deteriorate, Charles Koch wrote in the letter. As a result, he is unable to be involved in business and other organizational activities.

The Koch brothers have used their political network to move the Republican Party and the whole of American politics in a libertarian direction, and David Koch was the Libertarian Partys vice presidential nominee in 1980. They have founded and heavily funded multiple groups backing the Republican Party and conservative ideas, becoming the bte noire of many Democrats .

The two brothers are among Americas wealthiest men, with Forbes magazine recently estimating their net worth to be about $52 billion each . Koch Industries is the United States second-largest privately held company and makes everything from toilet paper to fertilizer.

Mark Holden, the chairman of Kochs donor network, confirmed David Kochs resignation from Americans for Prosperity.

Due to health reasons, David Koch will be resigning from the board of Americans for Prosperity Foundation, Holden said. We greatly appreciate his vital role on the board and all that he has done to help us build a strong foundation for our future success.

David Koch has also donated hundreds of millions to apolitical causes, including $185 million to the Massachusetts Institute of Technology, his alma mater, and $150 million to Memorial Sloan Kettering Cancer Center in New York City.

Its unclear how David Kochs retirement will change the brothers political activities, if at all. In an April interview with the conservative Washington Examiner , Charles Koch described the difference between the two brothers: David is a much better engineer than I am and is much more into the arts and social life. Obviously hes got to be or he wouldnt live in Manhattan. And David is much more into elective politics than I am.

CORRECTION: A previous version of this story misstated the estimated wealth of the Koch brothers as $80 billion each.