Biden and Harris vs Trump.

Since this is the closest I have ever realistically felt like Biden might withdraw in an absurd news cycle, here's Times/Siena polling on Harris in Pennsylvania and Virginia. Though note these are within the margins of error.

I think it is hard to say on polling alone that Harris will substantially change the state of the race (which is actually still fairly close). It is hard to poll hypotheticals and her polling is noisy.

You don't have to go too far back to consider how Hillary Clinton polled before she became an actual presidential candidate (she polled pretty well!).