The flagship organization of the political network backed by billionaires Charles and David Koch (kohk) is ditching campaign ads and building an army of conservative activists to shape the final days of the midterm elections.

Americans for Prosperity has sent more than 500 workers into 35 states in the past few years to shape how voters see issues such as health care and the economy.

Americans for Prosperity has become one of the conservative movement’s best organized outside groups and has a political footprint unmatched among GOP-affiliated operations.

The group’s big spending in Senate races has attracted national attention. But it’s the less noticed field efforts that could have greater impact and help reshape the Republican Party heading into the 2016 presidential election.

The Koch brothers fortune comes from the Kansas-based petrochemical conglomerate Koch Industries.