She stepped up to save the Party 107 days before the GE.

Her previous high polling period reflects her campaigns time rallying the base's support for her/ticket and something in polling called enthusiasm bias where more Democrats were willing to participate in polling during the month the DNC Convention is held. The GOP sees the same thing, though usually to a lesser extent.

Once into 'swing voters' season, she had a bit of a slower start and we are seeing Trump benefiting from that decision. 

To help clarify just how much of an uphill battle Harris faces and why she needs everyone's unwavering support no matter what the polls and media spin = the shortest winning presidential campaign was 357 days or  11 months, 22 days from announcing to GE by Ronald Reagan. 

So give Harris credit & grace while staying the course.

Vote early if you can!