I sell parody designs on Redbubble and Teepublic and have received three cease and desist letters over the past two years. Wanted to share my experience because the fair use analysis for parody merchandise is more nuanced than most people realize.
The Supreme Court case that matters most here is Campbell v. Acuff-Rose Music from 1994, which established that commercial parody can qualify as fair use. The court said that the more transformative a work is, the less the other fair use factors weigh against it. Parody specifically is given protection because it needs to reference the original in order to make its commentary.
However, and this is the part most t-shirt designers miss, there is a crucial difference between parody and satire in copyright law. Parody comments on the original work itself. Satire uses elements of the original to comment on something else entirely. Courts are much less sympathetic to satire claims because the satirist does not need the original work to make their point. If your design uses a brand logo to make fun of politics rather than making fun of the brand itself, your fair use argument is significantly weaker.
In my three cases, the one design that clearly parodied the brand itself was the one I successfully defended. The other two were more satirical in nature, using brand imagery to comment on unrelated topics. I ended up removing those voluntarily rather than risk litigation. The cost of defending a copyright lawsuit, even one you might win, can easily reach 50,000 to 100,000 dollars. That is not a risk most individual sellers can afford to take.