  A measurement is presented of the effective leptonic weak mixing angle
($\sin^2\theta^{\ell}_{\text{eff}}$) using the forward-backward asymmetry of
Drell-Yan lepton pairs ($\mu\mu$ and ee) produced in proton-proton collisions
at $\sqrt{s}=$ 8 TeV at the CMS experiment of the LHC. The data correspond to
integrated luminosities of 18.8 and 19.6 fb$^{-1}$ in the dimuon and dielectron
channels, respectively, containing 8.2 million dimuon and 4.9 million
dielectron events. With more events and new analysis techniques, including
constraints obtained on the parton distribution functions from the measured
forward-backward asymmetry, the statistical and systematic uncertainties are
significantly reduced relative to previous CMS measurements. The extracted
value of $\sin^2\theta^{\ell}_{\text{eff}}$ from the combined dilepton data is
$\sin^2\theta^{\ell}_{\text{eff}} =$ 0.23101 $\pm$ 0.00036 (stat) $\pm$ 0.00018
(syst) $\pm$ 0.00016 (theo) $\pm$ 0.00031 (parton distributions in proton) =
0.23101 $\pm$ 0.00053.
