  We present Raman scattering experiments in ${\rm La_{2-x}Sr_xCuO_4}$ single
crystals at various doping levels x and compare the results with theoretical
predictions obtained assuming an interaction mediated by spin and charge
fluctuations. The light-scattering selection rules allow us to disentangle
their respective contributions. We find that the glue spectral function is
spin-dominated at low doping while the contribution of charge fluctuations
becomes dominant around optimal doping. This indicates that the fluctuations of
a nearly ordered state with coexisting spin and charge order support the
superconducting pairing.

Superconductivity