The Communist Party USA.
Now, it's true that no Communist government has made much progress toward the withering away of the state. And that economic activity has been mostly controlled by government rather than by the cooperative organizations which (under true communism) would be autonomous. However, the Republicans in Washington haven't made any more progress toward increasing states' rights, or toward fiscal responsibility in government.
If you'll only belong to a political party which lives up to its proclaimed ideals, you're not going to have much choice. The Anti-Masonic Party's first (and only) Presidential candidate was a Mason. During election seasons, the Green Party generates more campaign trash in my neighborhood than any other party.