Gallup polling found a huge shift in party preference over the course of 2021, from a 9-point Democratic advantage in the first quarter to a 5-point Republican edge in the fourth quarter.

Why it matters: It's the biggest swing in one calendar year for Gallup's 30 years of tracking.

Reality check: American politics is all about volatility.

What's happening: The swing is tied to the popularity of the party heads, Gallup senior editor Jeff Jones writes.

• Trump was at his low when he left office, and President Biden's popularity started sliding in the summer.

Between the lines: Independents are still America's largest political group, Gallup notes.

• In 2021, 29% of U.S. adults identified as Democrats, 27% as Republicans and 42% as independents.
• An equal split of independents leaned D (17%) or R (16%).

The findings: In Q1 last year, 49% of U.S. adults identified as Democrats or leaned D. 40% identified as Republicans or leaned R.

• By Q4, 42% were Ds or leaners, and 47% were Rs or leaners.

Both the 9-point D advantage in Q1 and 5-point R edge in Q4 are among the largest Gallup has measured, going back to 1991.

• Jones tells me this was the biggest swing in a single calendar year: "There were a couple that were close — 1995 went from +5 Rep in Q1 to +7 Dem in Q4 ... 1992 went from +1 Dem in Q1 to +12 Dem in Q4 ... 1994 went from +7 Dem in Q1 to +3 Rep in Q4."

What we're watching: The GOP advantage may already be eroding.

• Gallup's December tracker showed the two parties even — 46% Republican/leaning and 44% Democratic/leaning.