The data isn't irrelevant at all - you know what else data can't really predict? whether or not if Sanders wins the election whether he will literally levitate.

This is just silly and sarcastic. I've provided plenty of reasons, let me provide some more specificity.

There is a large amount of data (not n=2) that suggests the ideology of voters is strongly predictive of whom they vote for at all levels (local, state, national)

Wrong, not national. For non-incumbent presidential elections n=2 (Obama/McCain, Gore/Bush) as opportunities post the invention of this measure for testing it. The others are confounded by incumbents, which itself is largely predictive. You have too little data to control for this on the relevant fron. Moreover, those ideologically far-economic-left candidates are confounded by not have concomitant economic support, which Sanders would have at the money-saturation level of presidential elections. Seriously, this is a huge factor, especially when you bring up...

There's a heap of research on negative ads, and how people with certain ideologies respond when they see an attack on a moderate vs. an attack on someone who is even more to the left of them.

Indeed, but do you know where money becomes less important? Presidential elections: one candidate at $1.5 bil spent vs. $1.0 bil doesn't make much difference because we're past saturation. Not to mention his individual campaign promises poll well, ideologically. If you want to test ideology more robustly, without appealing to literally n=2, then you need to dig deeper and more comprehensively into polling, which is something you have not done.