NEW YORK - Billionaire conservative icon David Koch is stepping down from the Koch brothers' network of business and political activities.

The 78-year-old New York resident is suffering from deteriorating health, according to a letter from older brother Charles Koch sent to company officials Tuesday.

David Koch is stepping down from Koch Industries and the Koch political and policy network due to declining health. pic.twitter.com/xnDwAWfqaU — Leigh Ann Caldwell (@LACaldwellDC) June 5, 2018

Charles Koch wrote: "We are deeply saddened by this, as we miss David's insightful questions and his many contributions to Koch Industries."

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David Koch is leaving his roles as executive vice president and board member for Koch Industries and chairman of the board for the Americans For Prosperity Foundation, the charity related to Koch brothers' primary political organization.

Charles Koch will continue to serve as CEO of Koch Industries and the unofficial face of the network.

Democrats have demonized the brothers for their outsized influence in conservative politics.