It is true that the polling of Harris, both as vice president and versus Trump, is likely to be at least somewhat influenced by the unpopularity of Biden. It's possible that her popularity could rise now as a presidential candidate facing Trump. However, as I noted she only has around 100 days to change an unfavourable perception of her that has existed for years at this point. I doubt that is enough time for a significant shift in her public image, particularly since she is still the current vice president and continues to be directly associated with Biden's presidency.

I’m also curious to see examples of the American left refusing to acknowledge the potential of a second Trump term.

I can't really answer this in the same way I have answered your other questions. It's my personal opinion, based on the narratives and discourse I see in the American media and among left-leaning Americans on social media. After the shock of the 2016 election, people seem to be retreating further into their left-leaning bubbles and refusing to accept that this can and likely will happen a second time, rather than engaging with the other side and trying to make sense of the country they live in. The 2024 campaign has been an absolute disaster for the left so far, yet now that Biden has removed himself from the race there seems to be another wave of delusional celebration from people who think Harris will beat Trump by virtue of being younger and not Biden.