Check individual polls from @PpollingNumbers or @Politics_Polls on twitter, or any other account you follow. It's a general trend going on since late August.
I won't go quoting each and every poll, so more practically, let's talk aggregates. Nate Silver's model shows Harris numbers worsening. From almost 4 percentage points ahead at national level weeks ago to 3.2 to 2.9 now. Her lead is down as well at the more crucial state level, almost all of them.
https://www.natesilver.net/p/nate-silver-2024-president-election-polls-model
Now bear in mind the following:
1-
Trump's effect: Trump beat his polls both in 2016 and 2020 by several percentage points in average. Biden won 'cause he had a comfortable lead. yet look at the numbers back then. Biden was ahead by 7-8 points nationally, he won by 4.5, in PA was ahead by 4.5 , won it by 1.3 , Trump was ahead in OH by 1 and won by 8!! If this repeats itself this year, it will be a landslide!!
2-
Electoral college bias: The EC is biased against the democrats. That is, they can win the popular vote by 1 or 2 points nationally yet lose the more crucial EC. According to Silver Democrats need to be ahead by 4 or more percentage points to avoid such National/EC split. Harris again is ahead by just 2.9 and worsening. Biden survived by being 7 points ahead.
Even if 1 doesn't happen this year, fingers crossed, Harris still has to deal with 2, and with her declining numbers. But she was never an inspiring or amazing debater/orator.