A slogan seen on t-shirts and bumper stickers. It was made by someone who was attempting to be clever, and failed epically in that attempt. It's only used by people whose knowledge of politics is so dismally lacking, that they must boil down the entire American political system into the basic Good vs. Evil of the Harry Potter books, and associate the bad guy with the political party that they've been told to hate, even though they don't actually know why they hate it.
Anyone who, (a) has read the Harry Potter books, (b) has at least an elementary knowledge of politics and history, and (c) has an IQ above room temperature, should be able to figure out that Rowling clearly based Voldemort and his Death Eaters on Adolf Hitler and the Nazi party. Both want a totalitarian government, both wish to oppress, persecute, and exterminate those that they see as being born genetically inferior based on the "purity" of their blood. They blame all of their problems on a certain group of people. They manipulate school curriculum in order to indoctrinate children against this group. The "pure blood" motif is also very similar to the outdated British Nobility system, of which the few that remain still cling to.
Even though you see "Republicans for Voldemort" crap everywhere, Voldemort really has nothing whatsoever to do with the Republican party of the U.S., or any other modern political party. If a comparison MUST be made, the Republicans are most like Scrimgeour: determined to protect us from terrorists, but perhaps a too heavy-handed with security. Meanwhile, Democrats are more like Fudge and Umbridge: in denial that such a big threat exists, over-regulating everything with too many rules, and dominating most media outlets.