tl;dr A US political party is more like a European coalition government than a European political party.

US political parties try to form tents that various subgroups can join under. Usually, some sort of compromise is formed among the various participants. One break down in the Democratic Party tent was over Israel/Gaza, another was over pro-tech/anti-tech. Simultaneously, there were factional wars over redistribution and immigration in both parties. These are two such but perhaps not even the biggest two such things. Inflation and government spending were another. And Biden's competence was also in question.

Every faction is likely convinced their own support is what would have turned the tide because it is somewhat true, except for the property that they're linked. e.g. pro-Gaza positions are also usually anti-tech so depending on how much you aim to get more Gaza supporters you also lose pro-industry people. There are many things like that.

A US politician will therefore try to walk the line of support to get elected. For example, you'll see a substantial change in Sen. Elizabeth Warren's positioning over time. Notably, she is currently actively attempting to reduce housing construction by corporations - a position she has not been historically associated with - because this polls very well among Americans (who, for the most part, believe that building new expensive housing makes all housing cost more).