What mechanism supports a white dwarf against gravity before mass gain?

Electron degeneracy pressure.

A white dwarf represents the final, dense state of stellar evolution for lower-mass stars, characterized by the cessation of nuclear fusion. Despite gravity attempting to compress it further, the white dwarf maintains a stable size because it is supported by electron degeneracy pressure. This pressure is a quantum mechanical effect where the densely packed electrons resist being squeezed into the same quantum state, thus providing a robust outward force that counteracts the inward pull of gravity. This resistance is effective only up to a specific mass limit, the Chandrasekhar limit. Beyond that mass, gravity overcomes this quantum mechanical resistance, leading to core collapse or, in the case of a white dwarf, initiating thermonuclear runaway.