  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 by the topological
feature of quantum chromodynamics (QCD). 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, within $\sim$2$\sigma$ significance, if the
axial charge per entropy density ($n_5/s$) and the QCD vacuum transition
probability are system independent. This expectation is generally verified by
the Anomalous-Viscous Fluid Dynamics (AVFD) model. While our estimate provides
an approximate "experimental" baseline, theoretical uncertainties on the CME
remain large.
