Trump is targeting mostly far-right evangelicals who have a common vision on what they want the country to look like. He has a lot of energy when doing so, and because of how similar their interests are he could get away with all sorts of stuff and they would still vote for him.
Harris (and Democrats in general) is the only alternative mainstream candidate that everyone else has, and that "everyone else" consists of all sorts of people with conflicting interests: liberals, neoliberals, centrists, progressives, leftists, different religious groups or cultures, varying economic demographics, racial minorities, LGBTQ, and immigrants for instance. They're trying to appeal to all of them at once, but because they don't have a shared vision, nobody is happy and they get more scrutinized. To make at least some of them happy, they need to focus on certain groups and deprioritize the interests of other groups. However, once they do that then the groups they deprioritize get angry since they no longer have representation, and the groups that are still there remain skeptical because of the history of not working for their interests in the past.
The advantage that third parties like PSL have is that from the start, they're trying to appeal to a specific group of people with a common vision like Trump is instead of trying to play both sides with conflicting groups and making nobody happy. The problem (aside from the election duopoly bought out by corporations) is that they are a very small political minority so they have no real chance of winning the election without winning over people from other groups which is a challenge, especially when there are many more unknowns when it comes to progressing than there are when it comes to reverting to a previous state so there is more fragmentation due to those sort of disagreements.