“Facing a potential primary challenge on her left from Democratic Rep. Ruben Gallego, Sinema stood a real chance of losing renomination if she sought reelection as a Democrat (she might’ve been in trouble against a more center-left Democrat, too, like Rep. Greg Stanton). Tellingly, Yoshinaka’s study found the prospect of facing a highly competitive primary in one’s own party can play into leaving that party.”
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“if Sinema’s chances of winning a Democratic primary were mediocre at best, it’s unclear how much stronger her path would be as an independent. It’s hard to imagine Republicans deciding not to field a major candidate against Sinema even if she’s an independent, but it’s possible she is hoping that the potential complications of a three-way race discourage a high-profile Democrat like Gallego from running. In that scenario, perhaps Democrats line up behind her in a head-to-head race against a Republican.
However, Gallego has already responded to Sinema’s switch by sending out fundraising texts that say he’s considering a Senate run. Now, Sinema might be able to put together a mishmash coalition of Democrats, Republicans and independents to win a three-way contest. After all, that Suffolk poll found that Republican likely voters also had a slightly more positive view of her than Democrats (35 percent favorable, 40 percent unfavorable), while independent likely voters had net-positive attitudes (42 percent favorable, 27 percent unfavorable). And she could attract plurality support if Democrats and Republicans nominate candidates who are viewed as too far left or right. That’s a possibility, too, as Gallego is a member of the Congressional Progressive Caucus, and Arizona Republicans just nominated far-right contenders Blake Masters and Kari Lake in the 2022 Senate and gubernatorial races, respectively.
But Sinema could certainly also find herself running in last place. Yoshinaka’s study found party switchers suffer an electoral penalty in their first general election after switching, with an average decline of 4 to 9 percentage points in vote share. Having upset Democrats, Sinema might lose most of their support to the Democratic pick, and there’s no guarantee that many Republicans back her over their party’s nominee, even if that candidate is highly problematic.”
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“The difficulties Sinema is likely to encounter speak to why senators rarely switch parties, and why it’s even more unusual for them to become — and stay — independent. Sinema is just the 10th senator since 1951 to formally switch parties while in office”
https://fivethirtyeight.com/features/why-kyrsten-sinema-left-the-democratic-party/