To discuss the relationship between voltage and current, we just have to use Ohm's Law.
According to this law, the voltage (V) and the current (I) have a direct relationship, which means if the voltage increases then the electrical current also increases.
The evidence to support this is Ohm's Law itself because it's being proven and generalized already.
The relationship between voltage and current is causal because if you apply a certain voltage, then you will have an electrical current.