Three days ago, The Post wrote about a few reasons the Georgia Senate runoff – and whether Democrats’ majority would grow to 51-49 – mattered, practically speaking. One of those reasons? The possibility of a party switch.

Sen. Kyrsten Sinema

That has already come to pass: Sen. Kyrsten Sinema, D-Ariz., announced in interviews, a video and an op-ed Friday that she will re-register as an independent. She becomes the first senator to leave her party since Sen. Arlen Specter, D-Pa., in 2009.

Like Specter, Sinema looked set to face an arduous primary if she sought reelection with her former party, given the maneuvering of Rep. Ruben Gallego, D-Ariz., to run against her. So the move makes some sense for her personally.

But what about the political impact more broadly?

The first thing to note is that it remains unclear whether Sinema will continue to caucus with Democrats, as two other independents in the Senate do. When asked about this, Sinema spokesman Pablo Sierra-Carmona said merely that “she intends to maintain her committee assignments from the Democratic majority. She has never and will not attend caucus messaging or organizational meetings.”

That sounds like she would effectively caucus with Democrats – that is, align with them for purposes of organizing the Senate – but for some reason is avoiding saying so directly. And she has said she’s not sure whether her desk will remain on the Democratic side of the Senate. Asked by CNN’s Jake Tapper whether her move would change the balance of power in the Senate, she responded, “that’s kind of a D.C. thing to worry about.”

This question doesn’t immediately matter when it comes to whether Democrats will retain the Senate majority, but it does matter. They will have at least a 50-49 edge as long as Sinema doesn’t caucus with the GOP. But if her plan is to leave the Democratic caucus, that would make Sen. Raphael G. Warnock’s, D-Ga., win in Tuesday’s runoff potentially hugely significant.

Of course, we’ll never know what Sinema might have done if Warnock hadn’t won. At that point, a party switch without caucusing with Democrats would have meant shifting the Senate majority to Republicans. (That has happened before; Vermont Sen. Jim Jeffords left the GOP to become an independent who caucused with Democrats 21 years ago, flipping the Senate majority.) Her calculus might have shifted in that scenario: However little Democratic support she’d get in a potential 2024 reelection bid, imagine her trying to appeal to any of the Democrats who elected her in 2018 after having handed the Senate majority to the GOP.

Indeed, the fact that Sinema waited to do this until after the Georgia runoff suggests she at least wanted to see how it shook out.

It also could matter for committees. Democrats will have a majority regardless, but their composition is subject to negotiations at the start of the new Congress.

And there’s also what it could mean for potential vacancies in Congress over the next two years. If Sinema didn’t caucus with Democrats, it would mean that even one vacancy in the wrong place – rather than two – could feasibly hand Republicans the majority. Currently, 11 Democratic senators come from states with a GOP governor who could appoint a Republican replacement.

Now let’s turn to Sinema’s own political future.

As noted above, this move makes sense for Sinema if she runs again, and runs as an independent. She has alienated many Democratic voters by resisting moves like nixing the filibuster and by standing in the way of some key agenda items for President Biden. It’s to the point where she would have had a very difficult time winning a primary, if she ran.

A September poll from AARP showed just 37 percent of Arizona Democrats had a favorable opinion of her, compared to 57 percent who had an unfavorable one. And the limited, very early polling of a Sinema versus Gallego matchup suggests Gallego might indeed be the favorite.

She’s not particularly popular among Republicans or independents, either, but if she were to run as an independent in a three-way race, it might open the door a little wider. That goes particularly if Arizona keeps nominating the kind of far-right Republicans they put up for statewide this year.

Also consider what Democrats do now. If they run someone like Gallego against an independent Sinema, the general election will feature two candidates who are, or were up until recently, Democrats. That risks splitting Democrats’ votes and possibly opening the door to Republicans – even a more extreme Republican than may otherwise have had a shot. Sinema has put her party in a potentially difficult spot. Democrats don’t support candidates against independent Sens. Angus King of Maine and Bernie Sanders of Vermont, but each of them are much more reliable votes for the party. With Sinema, they’ll have a difficult time dissuading liberals from mounting a candidacy.

Then there’s the matter of what the change means for how she votes in the Senate. Sinema has maintained that she won’t change her approach. “I intend to show up to work, do the same work that I always do,” she told Politico. “I just intend to show up to work as an independent.”

But everyone has their political prerogatives to mind. And if Sinema views this as freeing her up even a little from needing to appeal to the Democratic base, that could matter in the Senate. Democrats already have to contend with one of their majority-making votes coming from Sen. Joe Manchin III, D-W.Va., who comes from the second-Trumpiest state in the country. Even though they might have an effective 51-49 majority in the chamber, they still need either Sinema or Manchin on most close votes. And Sinema has previously shown she’s plenty willing to buck her party, even to her potential own political detriment.

In other words: Stay tuned. The incoming 118th Congress and the 2024 election just got a little more interesting.