The strong nuclear force.
The strong nuclear force is an attractive force which holds protons to each other. neutrons to each other and protons to neutrons. It falls off much more rapidly than Coulomb's law, so it dominates if the particles are close together, but once they are separated slightly they will be repelled (at least the protons will) from each other very powerfully at the distance where the Coulomb is stronger than the Strong.
Conversely, as in fusion, when it is necessary to heat hydrogen to very high temperatures to get the protons near enough together.
A more thorough treatment requires studying quantum tunnelling.