  We build template maps for the polarized Galactic--synchrotron emission on
large angular scales (FWHM =~7$^\circ$), in the 20-90 GHz microwave range, by
using WMAP data. The method, presented in a recent work, requires a synchrotron
total intensity survey and the {\it polarization horizon} to model the
polarized intensity and a starlight polarization map to model polarization
angles. The basic template is obtained directly at 23 GHz with about 94%
sky--coverage by using the synchrotron map released by the WMAP team.
Extrapolations to 32, 60 and 90 GHz are performed by computing a synchrotron
spectral index map, which strongly reduces previous uncertainties in passing
from low (1.4 GHz) to microwave frequencies. Differing from low frequency data,
none of our templates presents relevant structures out of the Galactic Plane.
Our map at 90 GHz suggests that the synchrotron emission at high Galactic
latitudes is low enough to allow a robust detection of the $E$--mode component
of the cosmological signal on large--scale, even in models with
low--reionization ($\tau = 0.05$). Detection of the weaker $B$--mode on the
largest scales ($\ell < 10$) might be jeopardized unless the value $\tau =
0.17$ found by WMAP is confirmed, and $T/S > 0.1$. For lower levels of the
gravitational--wave background the $B$--mode seems to be accessible only at the
$\ell \sim 100$ peak and in selected low--synchrotron emission areas.
