Personally, I think it can be a very hard rule to follow, especially with politicians.
Much of Scripture teaches about good and bad, right and wrong. If a person can't tell the difference between them and then act accordingly, they are often called fools and wrong and bad will find them, perhaps to their eternal discomfort.
We weren't created to sin or to be fools. I also don't think we are supposed to be so "not of the world" that we are of no use in the world. What we think and do has meaning, for us personally, as well as for those who depend upon us.
I could be wrong, but I don't think the following violates the "Thou shalt not judge" rule:
"If someone lies, they are a liar and a fool. Don't vote for them. If you do, you may not be a liar, but you would be a fool."
Depending on my intent, I also don't think it would be sinful to say that to someone I care about who voted for the one known as "obama", if my heart was right when I did it.
What followed saying it might be though, but maybe not for much longer. A few I know quit fighting and now just nod. There's hope!