If he wins the nomination, 15+ for sure. If he was able to successfully sell himself to the Democratic apparatus, he'd get at least 43% in the GE. The country is too polarized for him to lose in a true landslide.
I fundamentally disagree with the "country is polarized" meme. The country is definitely polarized on one side; about 40-42% of voters would never even consider voting for any Democrat, regardless of how great the Democratic candidate was or how terrible the Republican candidate was (in part, witness Trump, though Clinton was not a particularly amazing candidate on the Democratic side). But I am not at all convinced that the same holds true on the left. I think the hard floor for the Democrats is no more than about 30% or so. It is true that, due to demographic and identity (increase in self-ID'd liberals) changes, the floor for the Democrats is slowly growing while the floor for the Republicans is steadily shrinking, and it is also true that the Democrats typically win the sorts of voters who can go both ways overall even when losing elections nationally, but I think a truly terrible candidate for the Democrats, like Kanye, would be far more disastrous for the Democrats than a truly terrible candidate for the Republicans would be (is, perhaps, in the case of Trump) for the Republicans because the floor is much lower.
Never minding Clinton states that are actually swing-ish, like Nevada, Colorado, Virginia, Minnesota, New Hampshire and Maine, it is extremely difficult for me to imagine Kanye winning states like Delaware, New Jersey, New York, Connecticut, Massachusetts, Rhode Island and Illinois. There are way, way too many solid D voters (who last voted Republican in 1988 if not earlier or never at all) in those states who would never, ever consider voting for Kanye. They wouldn't necessarily vote for Trump, either (a sizeable chunk would, though), but they most likely would not see enough of a difference to turn out.
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