Which logical fallacy is set up by attacking an opposing argument that was never put forth?

This logical fallacy is called the straw man.

Explanation:

The straw man is a logical fallacy commonly used in highly emotional or polemical debates. It consists of attacking an argument that the opponent has never put forth. It is done by distorting the opponent's original argument, substituting it for a different one (a straw man) that is easier to attack and defeat. Refuting this new false argument is known as "knocking down the straw man." One way to do it, for instance, would be by quoting your opponent's words out of context, which allows for misinterpretation; one can also exaggerate or oversimplify the original argument. Anyhow, these strategies are considered fallacies because they do not refute the original argument at all, but merely offer an illusion of doing so.

Fallacies of omission.