A big part of the trick going on in polling is the shift from Registered voters to Likely voters.
There is no exact science to likely voters.
Initially, pollsters who want a certain outcome can adjust their sample results to characterize voters as “likely” according to a pre ordained outcome desired.
I am sure there is serious internal polling pressure to produce polls for Harris winning.
My own personal theory is that Harris was rushed out as the nominee because if she falters soon they will put in yet another candidate at the DNC. She is trying to prove that this radical coup against Biden was justified as are a number or Democrat traitors such as Pelosi and Schumer.