There's no great mystery about the motives behind this deliberate misnaming. "Democrat Party" is a slur, or intended to be, a handy way to express contempt. Aesthetic judgments are subjective, of course, but "Democrat Party" is jarring verging on ugly. It fairly screams "rat." At a slightly higher level of sophistication, it's an attempt to deny the enemy the positive connotations of its chosen appellation.
Digby caught a good example in John Boehner's speech on "civility" at the opening of the new congress last week. It's not just bad grammar, it's impolite-
You see, it actually isn't very civil at all to change the name of someone's else's political party against their will. In fact, it's universally considered rude and cretinous not to call people by the names and designations by which they wish to be called.
It may seem small but it's part of the Orwellian assault on language that conservative strategists have been implementing for years. Democratic politicians do us all a disservice when they let it slide in the name of comity. Boehner's remarks should've been followed by a rhetorical bitch-slap from a Democrat calling him on the hypocrisy of calling for civility while playing rude semantic games. It's going to take that level of awareness to put enough of a focus on the embarrass embarass republics, sorry republicans, into changing their ways.