Lisa has always been the moral and intellectual center of The Simpsons, which usually isolates her character.
Indeed, her know-it-all-ness can be annoying, but she espouses the true spirit of liberalism.
And that makes it all more upsetting for her when she discovers her newest best friend is a Republican in The Simpsons Season 25 Episode 6. But that doesn't mean they can't be friends, right?
What really worked about this episode was its tight focus on Lisa and the continuing problem of her loneliness, which was expertly captured in the opening scene of Lisa singing "One."
This set-up made it all the more exciting when she met Isabel in the library (similar to when she met Allison Taylor in season six, but without the initial competitiveness).
Setting up their political differences gave the show an excuse to trot out some political humor; although, the writers seem to be clearly more on Lisa's side than Isabel's.
The two got along so well that they decided to ignore their differing political views, each convincing themselves that the other is just going through a phase. This friendly spirit kept both girls from playing dirty in the election (although they were both tempted). It's a lovely lesson about how political beliefs don't necessarily make someone a bad person.
The story was both entertaining and thought-provoking, but what really made it work were the little bits of non-political humor sprinkled throughout.
Marge told Lisa that someone who loved her put butter on her veggies... then the scene showed us Mr. Bergstrom (Lisa's long-lost substitute) readying Lisa's cold dish. Then later, Isabel, an Argentinean Jew, told Bart exactly what "Ay, caramba!" means. Bart was shocked enough to exclaim, "Hot damn!" one more time for good measure.
And of course, there were some great The Simpsons quotes. Check em out.