  The PREX-2 and CREX experiments in Hall A at Jefferson Lab are precision
measurements of parity violating elastic electron scattering from complex
nuclei. One requirement was that the incident electron beam polarization,
typically $\approx$90\%, be known with 1\% precision. We commissioned and
operated a M{\o}ller polarimeter on the beam line that exceeds this
requirement, achieving a precision of 0.89\% for PREX-2, and 0.85\% for CREX.
The uncertainty is purely systematic, accumulated from several different
sources, but dominated by our knowledge of the target polarization. Our
analysis also demonstrates the need for accurate atomic wave functions in order
to correct for the Levchuk Effect. We describe the details of the polarimeter
operation and analysis, as well as (for CREX) a comparison to results from a
different polarimeter based on Compton scattering.
