  It is shown that the transition to the low temperature superconducting state
in a 3D metal at high magnetic field is smeared dramatically by thermal
fluctuation of the superconducting order parameter. The resulting
superconducting-to-normal crossover occurs in a vortex liquid state which is
extended well below the mean-field $H_{c2}$. Application to MgB$_{2}$ yields
good quantitative agreement with recently reported data of dHvA oscillation in
the superconducting state.
