  Despite being usually considered two competing phenomena, charge-density-wave
and superconductivity coexist in few systems, the most emblematic one being the
transition metal dichalcogenide 2H-NbSe$_2$. This unusual condition is
responsible for specific Raman signatures across the two phase transitions in
this compound. While the appearance of a soft phonon mode is a well-established
fingerprint of the charge-density-wave order, the nature of the sharp sub-gap
mode emerging below the superconducting temperature is still under debate. In
this work we use the external pressure as a knob to unveil the delicate
interplay between the two orders, and consequently the nature of the
superconducting mode. Thanks to an advanced extreme-conditions Raman technique
we are able to follow the pressure evolution and the simultaneous collapse of
the two intertwined charge density wave and superconducting modes. The
comparison with microscopic calculations in a model system supports the
Higgs-type nature of the superconducting mode and suggests that
charge-density-wave and superconductivity in 2H-NbSe$_2$ involve mutual
electronic degrees of freedom. These findings fill knowledge gap on the
electronic mechanisms at play in transition metal dichalcogenides, a crucial
step to fully exploit their properties in few-layers systems optimized for
devices applications.
