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How does an appeal to popular opinion become an ad populum fallacy?

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An appeal to popular opinion becomes an ad populum fallacy when it is used as a substitute for logical reasoning. In logic, a fallacy is defined as erroneous reasoning that may appear sound but is actually defective.12

The ad populum argument—meaning an appeal "to the people"—is classified as a material fallacy, specifically a fallacy of relevance. It occurs when an individual attempts to persuade others to accept a conclusion by appealing to popular attitudes, such as a shared dislike of injustice, rather than providing substantive logical grounds to support the claim. Because these premises do not provide rational support for the conclusion, the argument fails to meet the standards of validity, where the truth of the premises must logically guarantee the truth of the conclusion.32