Why is the practice of saying hurtful on true things about the other candidate in an election called “mudslinging”?

The practice of "mudslinging" is the process of deliberately damaging the reputation of another candidate during an election. This is mostly done by spreading negative information about the candidate, whether honest or dishonest.

The practice is called "mudslinging" as a metaphor. The negative information is compared to mud that is being thrown towards the opponent.

i also think it is c