According to Gallop the percentage of people who identify as republican, democrat, and independent are:

D- 27% R- 28% I- 42%

https://news.gallup.com/poll/15370/party-affiliation.aspx?fbclid=IwAR1pdFG3Jxo4JR9cWYpUDtoEeBzpmVBnYjVQIk1mp9_YzRl-7BKHEI7L7Y8

Yet many polls have samples which don't represent this at all.

Taking 3 polls (that have info on party affiliation) from the last week:

Reuters/IPSOS, Oct 20th-22nd, national poll:

D-44% (17% oversample)

R-37% (9% oversample)

I-12% (30% undersample)

https://e.infogram.com/7803c640-a1aa-406a-a7c4-bf9efee425d3?parent_url=https%3A%2F%2Fwww.mitchellrepublic.com%2Fnews%2Fgovernment-and-politics%2F6731229-Oct.-20-22-polling-shows-Biden-gains-in-most-areas&src=embed&fbclid=IwAR1GnwgvDSInXiJYulcn1VQvdk2IuNhmxTirPgWpprfey5Z3hpIUMAYcJ_k#async_embed

IBD/TIPP, Oct 21st, national poll:

D-38.0% (10% oversample)

R-32.8% (5.8% oversample)

I-28.8% (13.2% undersample)

https://www.investors.com/news/trump-vs-biden-poll-favors-democrat-after-debate-ibd-tipp-presidential-poll/?fbclid=IwAR2RO1cIpkD9omNXtK7l3XSKQ2JOqa1w4C2msVWXwroAsz_qZ39zAjUqe5E

The economist/YouGov, Oct 18th-20th, national poll:

D-38.1% (11.1% oversample)

R-28.1% (0.1% oversample)

I-33.1% (8.9% undersample)

https://docs.cdn.yougov.com/hxop5lpnlj/econTabReport.pdf?fbclid=IwAR0UDLij4pygrHEYlvTPapIbdkeXXqIJEZxjCQHgNB9X0pG6-VSCaHzutBU

If you take the YouGov poll, which has Biden at 47% and Trump at 40% and break it down by party you get:

Democrats: Biden - 85%, Trump - 4%, not sure -3%

Republicans: Biden - 5%, Trump - 86%, not sure - 2%

Independents: Biden - 39%, Trump - 42%, not sure 8%

Taking this party breakdown and then applying the Gallup party affiliations to it generates this result:

Trump = (0.04 * 0.27) + (0.86 * 0.28) + (0.42 * 0.42) = 0.428 = 42.8%

Biden = (0.85 * 0.27) + (0.05 * 0.28) + (0.39 * 0.42) = 0.4073 = 40.7%

The poll goes from Biden - 47%, Trump - 40% to Biden - 40.7%, Trump - 42.8%, and that's nation wide.

I know that pollsters use 'likely voters' as a metric rather than a direct proportion of each party from the population, but when new voter registrations are up substantially for republicans over democrats (see below) it seems that democrats are being oversampled in these (and many other) polls.

https://www.nbcnews.com/politics/2020-election/trump-s-winning-voter-registration-battle-against-biden-key-states-n1241674?fbclid=IwAR2103F5yUEZXBOKTRwcoWOa_Dzpgx4XDMWBY18SSOOAQNjKNXA7JK8Kmek