Certainly there exists a value X where 100-X percent of the population is not engaged at all and is little more than a source of random numbers or demographic counts.

If X is 10% then its an excellent measure of the will of the actively thinking electorate and its not the fault of the statistician that Americans don't care about politics.

If X is 50% or so then there's 4 semi-motivated voters per primary voter which is getting fuzzy.

Note that about 20% of the population vote in non-presidential elections and 50% vote in presidential elections. I'd propose that the delta is due to tradition and heavy social signalling to get out the vote, but that 30% don't really care and absent intense PR activity in support of voting, would not vote. Unfortunately 20% is too high to clearly support the first criteria and 20% is too low to support the second criteria.

I think its really annoying that there is no mathematically trustworthy answer to is 9% enough. Annoyingly its probably good enough to not be ridiculously far off and bad enough to not be correct a significant amount of time.

It's 9% consisting of the members of two private clubs who have thrown up massive barriers to the participation of people who don't agree with either of their programs and who don't have to be active or engaged to be counted, just to vote correctly. Ideally, I'd think that the parties would want to an even smaller percentage to make the choice for the rest of us. 9% of the population have proved very difficult to wrangle this time around, with a near total failure on one of the sides.

Either that, or the best candidate for president out of 300 million is the wife of the president three terms ago. Bush, Clinton, Bush, Obama, Clinton? This has nothing to with issues.