  The emission line ratios [OIII]5007/H-beta and [NII]6584/H-alpha have been
adopted as an empirical way to distinguish between the fundamentally different
mechanisms of ionization in emission-line galaxies. However, detailed
interpretation of these diagnostics requires calculations of the internal
structure of the emitting HII regions, and these calculations depend on the
assumptions one makes about the relative importance of radiation pressure and
stellar winds. In this paper we construct a grid of quasi-static HII region
models to explore how choices about these parameters alter HII regions'
emission line ratios. We find that, when radiation pressure is included in our
models, HII regions reach a saturation point beyond which further increases in
the luminosity of the driving stars does not produce any further increase in
effective ionization parameter, and thus does not yield any further alteration
in an HII region's line ratio. We also show that, if stellar winds are assumed
to be strong, the maximum possible ionization parameter is quite low. As a
result of this effect, it is inconsistent to simultaneously assume that HII
regions are wind-blown bubbles and that they have high ionization parameters;
some popular HII region models suffer from this inconsistency. Our work in this
paper provides a foundation for a companion paper in which we embed the model
grids we compute here within a population synthesis code that enables us to
compute the integrated line emission from galactic populations of HII regions.
