As with all the discussions on national polls, it’s necessary to point out that national polls have no meaning. We don’t have national elections.
Blue states are gonna blue state and red states are gonna red state, so the President will be decided by a handfull of states that don’t reliably vote one way or another, and frankly, we just don’t have enough good data on a Harris nomination to say one way or another.
Looking at the notable states I’ve looked at before, this data is from before Biden dropped out.