More precision on income and voting?

Jeff writes,

Some questions of interest, picking up from your trio of income/state winners graphs:

At what income cutoff ($) would Dems and Repubs tie in electoral votes? What national income percentile is this?

What if you take the top x% within each state by income? (How low do you have to go before the Dems can win?)

My reply: I imagine the threshold would be close to the 50th percentile, just by symmetry since each party got about half the vote, and when we last looked, the electoral college had little partisan bias (i.e., the seats-votes curve goes close to 50/50; see here)..

But I’m wary of trying to get a more precise answer because this would lean too heavily on the assumed linearity of the income predictor in the model. Linearity fits reasonably well, but still, this is the sort of modeling exercise I wouldn’t trust. I certainly wouldn’t trust the linear model on theoretical grounds, given that the categories are just the arbitrary divisions used by NES.