Arizona was home to several competitive statewide contests this year, many of which had national implications. It stood to reason that Sen. Kyrsten Sinema would be actively involved in the elections, hitting the campaign trail and rallying support for her allies.

But she didn’t. Sinema kept a noticeably low profile in her home state, expressing apparent indifference to the outcomes of the critically important races. Democratic Rep. Ruben Gallego appeared on MSNBC after the elections, explaining, “We fought as a team in Arizona and we won. Sen. Sinema was nowhere to be found. At all.”

A month later, the senator’s partisan passivity can be seen in a new light: Sinema didn’t seem to care about the Democratic Party, its candidates or its interests because she’s no longer an actual Democrat. The Arizona Republic reported this morning:

Sen. Kyrsten Sinema said Friday she is leaving the Democratic Party and will formally become an independent in a move that more fully places her at the center of a narrowly divided chamber. She announced her decision in an opinion piece published Friday in The Arizona Republic.

Sinema’s announcement makes her the first sitting senator to change party affiliations since Pennsylvania’s Arlen Specter switched from Republican to Democrat in 2009.

Today’s news, however, isn’t quite as dramatic.

Sinema has left the Democratic Party and changed her affiliation to independent, but she’ll continue to caucus with Senate Democrats. She won’t be alone on that front: Sens. Bernie Sanders of Vermont and Angus King of Maine are independents who’ve partnered with Democrats for many years.

The circumstances with Sinema will be a little different — she’s said she won’t attend caucus meetings, though of late, she’s generally skipped the gatherings anyway — but in her op-ed for the Republic, the senator suggested her policy positions and voting habits won’t change. She also expects to keep the same committee assignments — as chosen by Democratic leaders — that she currently enjoys.

Sinema added in an interview with Politico, “I don’t anticipate that anything will change about the Senate structure.”

In other words, as of 24 hours ago, Democrats looked forward to having what was effectively a 51-seat majority in the chamber. As of this morning, that remains largely unchanged.

So why bother? If Sinema is going to vote the same way as she’s always voted, and if she intends to remain effectively a part of the majority conference, why make a formal switch from Democrat to independent?

The answer is likely related to her 2024 prospects.

To put it mildly, Sinema has not endeared herself to Democrats in her home state. Earlier this year, Data For Progress polled Arizona Democrats and found the incumbent senator trailing Gallego in a hypothetical primary match-up, 74% to 16%. That’s not a typo: She actually trailed the popular congressman by 58 points.

With this in mind, Sinema very likely realized that she had a decision to make: She could (a) retire; (b) take steps to repair the relationship with the Democratic voters who elected her in the first place; or (c) take her chances as an independent.

As a tactical matter, settling on the third option makes sense. Sinema was very likely to lose a Democratic primary, her voting record made a run as a Republican implausible, and she didn’t want to walk away from Capitol Hill altogether. Becoming an independent is a sensible strategy.

Indeed, it’s not unrealistic to think it might very well save her career. Democrats will likely think twice about running a candidate against her in 2024, fearing that splitting the center-left mainstream could leave the seat in Republican hands. Sinema’s announcement probably won’t dramatically alter the makeup of the Senate, but if it neutralizes an electoral threat, it’ll give the senator what she’s looking for: a re-election edge she wouldn’t otherwise have.

But as the political world digests the news, it’s also worth pausing to appreciate Sinema’s bizarre ideological trajectory. In the Bush/Cheney era, she was a liberal activist and failed Green Party candidate who railed against then-Sen. Joe Lieberman as a “pathetic” sellout.

Twenty years later, Sinema has done exactly what Lieberman did: become an independent while caucusing with Senate Democrats.