Our 2020 book is titled Divided We Stand: The 2020 Elections and American Politics. Among other things, it discusses the state of the parties.
Jeffrey M. Jones at Gallup:
On average, Americans' political party preferences in 2021 looked similar to prior years, with slightly more U.S. adults identifying as Democrats or leaning Democratic (46%) than identified as Republicans or leaned Republican (43%).
However, the general stability for the full-year average obscures a dramatic shift over the course of 2021, from a nine-percentage-point Democratic advantage in the first quarter to a rare five-point Republican edge in the fourth quarter.
In the first quarter of 2021, 49% of U.S. adults identified as Democrats or leaned Democratic, while 40% identified as Republicans or leaned Republican. In the second quarter, 49% were Democrats or Democratic leaners, and 43% were Republicans and Republican leaners. In the third quarter, 45% were Democrats and Democratic leaners, and were 44% Republicans and Republican leaners. In the fourth quarter, 42% were Democrats and Democratic leaners, and 47% were Republicans and Republican leaners.
These results are based on aggregated data from all U.S. Gallup telephone surveys in 2021, which included interviews with more than 12,000 randomly sampled U.S. adults.