  We consider the implications of low-energy precision tests of parity
violation on t-channel mediator models explaining the top AFB excess measured
by CDF and D0. Flavor-violating u-t or d-t couplings of new scalar or vector
mediators generate at one-loop an anomalous contribution to the nuclear weak
charge. As a result, atomic parity violation constraints disfavor at >3 sigma
t-channel models that give rise to a greater than 20% AFB at the parton level
for M_tt > 450 GeV while not producing too large a top cross-section. Even
stronger constraints are expected through future measurements of the proton
weak charge by the Q-Weak experiment.
