Decorate with red and/or blue banners, party hats and paper goods. You might serve cookies in the shape of donkeys and elephants or add some pizzaz by serving red velvet cake with blue icing. Plan a fun game or two, perhaps giving a small prize to whoever picks the winners of local elections.
Above all, keep it light and fun; no snide comments or displays of anger. Though the results are extremely important, it isn't necessary to make others uncomfortable about their allegiances.