  We propose a non-BCS mechanism for superconductivity in hole-underdoped
cuprates based on a gauge approach to the {\it t-J} model. The gluing force is
an attraction between spin vortices centered on the empty sites of two opposite
N\'eel sublattices, leading to pairing of charge carriers. In the presence of
these pairs, a gauge force coming from the single occupancy constraint induces,
in turn, the pairing of the spin carriers. The combination of the charge and
spin pairs gives rise to a finite density of incoherent hole pairs, leading to
a finite Nernst signal as precursor to superconductivity. The true
superconducting transition occurs at an even lower temperature, via a 3D
XY-type transition. The main features of this non-BCS description of
superconductivity are consistent with the experimental results in underdoped
cuprates, especially the contour plot of the Nernst signal.
