  Charge symmetry breaking contributions to the proton's neutral weak form
factors must be understood in order for future measurements of parity violating
electron-proton scattering to be definitively interpreted as evidence of proton
strangeness. We calculate these charge symmetry breaking form factor
contributions using chiral perturbation theory with resonance saturation
estimates for unknown low-energy constants. The uncertainty of the
leading-order resonance saturation estimates is reduced by incorporating
nuclear physics constraints. Higher-order contributions are investigated
through phenomenological vertex form factors. We predict that charge symmetry
breaking form factor contributions are an order of magnitude larger than
expected from naive dimensional analysis but are still an order of magnitude
smaller than current experimental bounds on proton strangeness. This is
consistent with previous calculations using chiral perturbation theory with
resonance saturation.
