Arizona Senator Kyrsten Sinema has announced she's leaving the Democratic Party and registering as an independent. It appears that Sinema will still be effectively caucusing with the Democrats (she's slated to keep her committee assignments), so to some extent this is just another performance and won't directly upset the balance of power in the upper chamber. Nonetheless, this is entirely in character for a woman whose politics are very slightly left-of-center but also mostly gibberish that she spins up as having a "maverick streak" and being too pure for the confines of the dreaded two party system.
I'm not exactly surprised by the news. Indeed, to some extent I think it's politically savvy. Sinema was a dead woman walking in a 2024 Democratic primary -- Mark Kelly demonstrating that one can be a normal Democrat and win in Arizona took away Sinema's only hope of staving off an inevitable progressive challenger. But formally identifying as in indie gives Sinema two big advantages.
Rhetorically, as empty as this gesture is, it still will appeal to a certain branch of voter (and, more importantly, pundit) who salivates over "no labels!" and "independence!". At this point, that cadre is the only base Sinema could ever hope to have, so she might as well juice them for all they're worth.
More practically, by laying the groundwork for an independent run now, Sinema arguably boxes off a Democratic challenge more thoroughly than she ever could within the party. Sinema has effectively flipped a dead man's switch and promised to run in 2024 even if it means splitting the vote with an actual Democratic nominee. Do Democrats feel confident they can win in Arizona even with Sinema peeling off a non-trivial part of the vote? I bet that prospect has Democratic strategists feeling real nervous. There are several instances (Alaska springs to mind) where Democrats have simply lined up behind independent candidates in states knowing that they can't win if they split the vote, and Sinema is trying to make Arizona one of them. (The wild card is if Republicans decide not to run a candidate against Sinema, but that strikes me as vanishingly unlikely. The infuriating truth of American politics is that even in her most asinine and frustrating moments, Sinema still isn't even close to being as bad as even a normal Republican, let alone an Arizona Republican).