  We study a finite temperature two-loop resummed effective potential in the
Abelian gauge theory. A tractable calculation scheme without using a
high-temperature expansion is devised. We apply it to the Abelian-Higgs model
and its extension to a minimal supersymmetric standard model-like model and
study the thermal phase transition. It is shown that our scheme improves the
previous results about 10% in the quantities relevant to the phase transition,
and its impacts on bubble dynamics could be even more sizable. It still holds
that the stop-stop-gluon sunset diagram enhances the strength of the
first-order phase transition even without the high-temperature expansion.
