  We discuss nonperturbative contributions to the 3-dimensional one-loop
effective potential of the electroweak theory at high temperatures in the
framework of the stochastic vacuum model. It assumes a gauge-field background
with Gaussian correlations which leads to confinement. The instability of
<F^2>=0 in Yang-Mills theory appears for small Higgs expectation value <\phi^2>
in an IR regularized form. The gauge boson propagator obtains a positive
momentum-dependent ``diamagnetic'' effective (mass)^2 due to confinement
effects and a negative one due to ``paramagnetic'' spin-spin interactions which
are related to the <F^2>=0 instability. Numerical evaluation of an approximate
effective potential containing these masses shows qualitatively the fading away
of the first-order phase transition with increasing Higgs mass which was
observed in lattice calculations. The crossover point can be roughly determined
postulating that the effective \phi^4 and \phi^2 terms vanish there.
