Measure P has no formal opposition and is supported by the Napa County Farm Bureau and the Napa Sierra Club.
Voters passed the previous initiative, Measure J, in 1990 to prevent urban sprawl and protect farmland and open space, proponents said. Measure J is set to expire in 2020.
"The power to preserve agricultural (land) and open space is currently in the hands of the people," supporters said in the voter information pamphlet.
"We still believe, as we did in 1990, that protection of Napa County's greatest agricultural resource is too vital to our future to be left in the hands of three county supervisors on any given Tuesday."
The provisions of Measure J require a vote of the electorate before any zoning changes are made in the Napa County Agricultural Preserve, established in 1968.
Under Measure P, agricultural, watershed and open space land could be converted to a site for state-mandated affordable housing or for solid waste facilities without a vote of the electorate if the Board of Supervisors approves.
Measure P requires majority approval.