  Isobaric $^{96}_{44}$Ru+$^{96}_{44}$Ru and $^{96}_{40}$Zr+$^{96}_{40}$Zr
collisions at $\sqrt{s_{_{NN}}}=200$ GeV have been conducted at the
Relativistic Heavy Ion Collider to circumvent the large flow-induced background
in searching for the chiral magnetic effect (CME), predicted to exist in
topological quantum chromodynamics. Considering that the background in isobar
collisions is approximately twice that in Au+Au collisions (due to the smaller
multiplicity) and the CME signal is approximately half (due to the weaker
magnetic field), we caution that the CME may not be detectable with the
collected isobar data statistics if the axial charge per entropy density
($n_5/s$) is system independent. We verify this general expectation by
Anomalous-Viscous Fluid Dynamics (AVFD). However, if the $n_5/s$ in isobar
collisions is 1.5 times that in Au+Au collisions, as AVFD indicates, then a CME
signal could be expected with a $\sim$2$\sigma$ significance.
