An ad populum comment is an appeal to popularity, also known as bandwagoning. Using this line of reasoning alone rarely lacks validity. Examples:
Anyone who cares about freedom would vote for the libertarian candidate.
If you disagree with my position, then you are a racist (or ageist, classist, sexist, etc.).
When asserting a position to find the common ground or to discredit an opposing view, choose words and phrases carefully. The following sentences may be regarded as improvements of the previous examples:
Since the libertarian candidate bases her platform on individual freedom, a vote for her would signal your support of a less restrictive government.
How would you distance your position from the apartheid policies formerly practiced in the Republic of South Africa?
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