  A blazar is believed to exist in every radio-loud quasar. This is expected in
a unified scheme where the differences in both optical and radio observations
of radio-loud quasars are the result of different viewing angles. We have
predicted that blazars may be detected using emission line ratio variations
caused by variable illumination of gas clouds in the broad emission line
region. In a spectroscopic search of 62 quasars at a redshift of about 2, we
have discovered large (>20%) variations of the emission line ratios, CIV/CIII]
or CIV/Ly$\alpha$, when compared with historical data taken over 10 years ago.
This result is consistent with our prediction, and thus supports the
unification scheme for radio-loud quasars.
