I don't think the candidate mattered that much. As far as I could tell, there were 4 big issues:
- Inflation/cost of living
- Immigration
- Abortion/reproductive rights
- Foreign wars
The Biden admin put the Democrats behind on 3 of the 4. Kamala also had the disadvantage where she had to defend the Biden admin but also position herself as a change candidate. Could also throw in how blatant the lawfare against Trump was and even the classified documents case became irrelevant because Biden did the exact same thing/fumbled that one as well.
Economy you could say they were going to be screwed either way. But from a political stand point they didn't play it well. Cost of day-to-day goods were too high while passing things like the "Inflation Reduction Act" and trying to gaslight to the electorate that it wasn't that bad. I think Kamala did the best that she could here, trying to pin it all on price gouging. She wasn't completely dismissing the pain people were feeling, but also wasn't just admitting responsibility or the inability to do anything. Ultimately you can only message so far to try to re-shape people's reality.
They did nothing on immigration, trying to be the anti-Trump, until the very end of his term and tried to use the border security bill as a political football. Trump then killed it, also as political football, but at that point the damage was done for the Democrats. They clearly weren't taking the border seriously. I don't think the candidate mattered here unless they fully denounced the Biden admin. Kamala being the border czar was worst of both worlds making it an impossible issue to climb.
Abortion/reproductive rights was their meal ticket to getting elected. Unfortunately for them as time goes on there is more distance between today and the Dobbs ruling and emotions fade. There is also more of a window for people to point to Democrats doing nothing to codify abortion.
Ukraine war it seems like most people don't care about anymore. In tandem with inflation/cost of living it is a losing issue to spend billions to fight a foreign war. But the Ukraine war doesn't seem like a big issue either way. The war in Israel though the Democrats have no real needle to thread. They have to support Palestinians, in order to masquerade being anti-war and fighting for the little guy, but also have to deepthroat Israel to not be anti-semitic. Republicans have no qualms about just deepthroating Israel. Just too impossible for a Democrat to win the issue while Biden is getting rail-roaded by Bibi.
For as seemingly erratic and unfocused Trump is, he has great political instincts and has the ability to make issues national issues. He did a great job making immigration a front and center issue again. He did a great job making the cost of living an issue. I don't think any other Democratic candidate would have been able to shift the national discussion going up against him.
There were a bunch of other stumbles by the Biden admin that made it difficult to run, but I don't think were polling issues. Stuff like his dementia, not having a real primary after he dropped out, not having Kamal do interviews, and the Trump assassination attempts put them on the back-foot and distracted from other selling points. But ultimately, the only selling points that seemed to be hitting was reproductive rights. Everything else was just an argument that the other guy was more bad.
Watching Kamala's campaign though, maybe they should have just nominated Liz Cheney. The DNC clearly loves her.