The fallacy known as poisoning the well involves presenting negative information about an opponent to preemptively discredit what he or she says.
For example, let’s say the president is running for reelection, and her primary opponent is an up-and-coming politician whom few in the public have heard of. Ahead of a televised debate, the president might poison the well by running negative campaign ads that highlight her opponents’ past transgressions or supposed moral failures so that viewers will not take what the opponent says seriously. And on the day of the debate, the president might further poison the well by making negative remarks about her opponent during opening statements.
Poisoning the well is very broad and has diverse applications. It can apply to any intentional advanced biasing. For example, if you say to your friend, “Don’t you think my wife is beautiful?” and then show him a picture of your wife, his answer will be affected by the phrasing of your question.