Charles Koch's network launches new fight to keep donors secret

A group tied to billionaire Charles Koch has unleashed an aggressive campaign to kill a ballot measure in South Dakota that would require Koch-affiliated groups and others like them to reveal their donors’ identities — part of a sustained effort by his powerful network to keep government agencies and the public from learning more about its financial backers.

Americans for Prosperity, the largest activist group in the policy and political empire founded by industrialist Koch and his brother, David, launched a coalition this year to fight Initiated Measure 22, which...