  We present for the first time a coherent model of the polarized Galactic
synchrotron and thermal dust emissions which are the main diffuse foreground
for the measurement of the polarized power spectra of the CMB fluctuations with
the Planck satellite mission. We produce 3D models of the Galactic magnetic
field including regular and turbulent components, and of the distribution of
matter in the Galaxy, relativistic electrons and dust grains. By integrating
along the line of sight we construct maps of the polarized Galactic synchrotron
and thermal dust emission for each of these models and compare them to
currently available data. We consider the 408 MHz all-sky continuum survey, the
23 GHz band of the Wilkinson Microwave Anisotropy Probe and the 353 GHz
Archeops data.}{The best-fit parameters obtained are consistent with previous
estimates in the literature based only on synchrotron emission and pulsar
rotation measurements. They allows us to reproduce the large scale structures
observed on the data. Poorly understood local Galactic structures and
turbulence make difficult an accurate reconstruction of the observations in the
Galactic plane. Finally, using the best-fit model we are able to estimate the
expected polarized foreground contamination at the Planck frequency bands. For
the CMB bands, 70, 100, 143 and 217 GHz, at high Galactic latitudes although
the CMB signal dominates in general, a significant foreground contribution is
expected at large angular scales. In particular, this contribution will
dominate the CMB signal for the B modes expected from realistic models of a
background of primordial gravitational waves.
