Comment by dhosek

There’s a partisan floor¹ that’s in the neighborhood of 25–30% where people will vote for their party no matter what. The 2004 Illinois Senate election was a good test case where when the Republican nominee went down in self-inflicted flames and was hurriedly replaced with Alan Keyes, who had no in-state constituency and, being Black (which, no doubt was the cynical reason for his recruitment), eliminated the racist choice. He gained a mere 27% of the vote which is likely the floor for a major party candidate in a contested broad-population election (smaller-level partisan elections, whether for single congressional seats or in small states, generally have demographically skewed populations which can result in wider margins).

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1. Although as I think about it, this may only be valid on the right side of the spectrum given the data points.