And 48% lean Democratic.

https://www.people-press.org/2016/09/13/2-party-affiliation-among-voters-1992-2016/

This is as of 2016. According to less-rigorous analysis the GOP number has decreased since then. But let's take the 44/48 split to be cautious.

Nobody's talking about winning over any of those 44% (even though some of those are only saying they "lean" Republican, so it's not out of the question especially when many of them are embarrassed by Trump).

The argument for moderates is that we need to maximize the fraction of the remaining 8% who votes for the Democrat.

The fact that partisan voters are so strongly polarized right now does not mean that there aren't also voters who can be swayed between the two parties. There are a lot of voters who are generally disengaged. There are a lot who voted for Trump because they liked his TV persona. There is a HUGE number of voters who claimed not to like either candidate in 2016 (as there are in most races) and said they were voting for the lesser of the 2 evils, the one they'd "rather have a beer with." These voters can add up, and I think it can be argued they're the ones who've decided every presidential race in our lifetimes.