No, it is never the middle that decides elections in the US. Democrats win when turnout is high, and Republicans win when turnout is low.
Know how AOC won her seat from Pelosi’s presumed successor who was considered unbeatable? She focused her campaign on unlikely voters. When she actually spoke to those people, they showed up to vote. That’s the path to victory for Democrats.
Hillary dominated with moderate voters in 2016. She was also an uninspiring elitist which led to terrible turnout. Biden wasn’t much better, but four years of Trump drove record breaking turnout,band Biden won.
It’s all about energy and engagement. Biden sure isn’t going to bring that next year, but maybe there is enough anti-Trump sentiment left to drive turnout. Of course Trump might not be the candidate. I think Biden might beat DeSantis, but not a slightly more obscure candidate like Nikki Haley.