This does not mean what you think it means, and is very often misused. "The exceptions prove that the rule is true" is obvious nonsense, the refuge of people who make an assertion which is easily disproven with obvious counterexamples. What this phrase actually means is that the exceptions prove that the rule exists, because if there were no rule then there would be nothing for the counterexamples to be an exception to.
This does not mean what you think it means, and is very often misused. "The exceptions prove that the rule is true" is obvious nonsense, the refuge of people who make an assertion which is easily disproven with obvious counterexamples. What this phrase actually means is that the exceptions prove that the rule exists, because if there were no rule then there would be nothing for the counterexamples to be an exception to.
The rule is proven by simply looking at voter breakdowns. Or marketing strategies.