That's simple. A principle is a proposition that has a non-trivial probability (even if small) of coming true or being implemented. Paranoia or delusion, otoh, is a proposition that, while seemingly possible and logical on its face, has zero probability of coming true or being implemented. Or stated differently, a principle is a belief checked by empirical evidence, delusion is belief held in spite of empirical evidence.
The third-partyism in the US fits the definition of delusion because the probability of a third party coming even remotely close to anything that resembles power is practically zero. Even in Western European parliamentary systems with much more developed environmental movement and left in general, the Greens barely hold to power and never attained more than 10 percent of the seats. Thus even under the best case scenario, the Greens can count on about 10 % of the votes - 40 percent short of having any chance of electoral victory under the US system. The only impact they can possibly have on the election is to split the vote and help the Repugs win. Believing that they can actually win in an election is being totally blind to empirical evidence - hence fits the definition of delusion.
Wojtek