> almost unheard of in the US for an elected politician to poll lower than 30%, it seems to be a floor

Alan Keyes apparently set the floor at 27%, which is not far below 30%

> Obama vs. Alan Keyes. ... Keyes was plainly, obviously, completely crazy. But 27% of the population of Illinois voted for him. They put party identification, personal prejudice, whatever ahead of rational judgement.

https://kfmonkey.blogspot.com/2005/10/lunch-discussions-145-...

https://en.wikipedia.org/wiki/2004_United_States_Senate_elec...

And lizardman's Constant is 4% https://en.wiktionary.org/wiki/Lizardman%27s_Constant