Why Kyrsten Sinema Left The Democratic Party

Remember when Georgia’s runoff gave Democrats 51 seats in the U.S. Senate? Well, Arizona Sen. Kyrsten Sinema said not so fast, my friend: This morning, Sinema announced she would change her party registration to independent and no longer identify as a Democrat moving forward. On paper, then, the next Senate’s makeup will be 49 Republicans, 48 Democrats and three independents.

Still, Sinema’s switch doesn’t necessarily change the Senate’s voting math. Sinema told the Arizona Republic she intends to mostly align with the Democrats and keep her committee assignments from that party (which Majority Leader Chuck Schumer confirmed he had agreed to honor). Alongside fellow independent Sens. Angus King of Maine and Bernie Sanders of Vermont, she appears set to give the party 51 seats when it comes to organizational questions, allowing them to hold majorities on committees. Considering that she may not alter her legislative habits, why did Sinema make this decision? Her reelection outlook in 2024 may help explain her move, although her centrist attitudes likely played some role.

To some, Sinema’s party switch might not come as a surprise considering her moderate reputation. After all, she has the second-most conservative voting record in the Senate among Democrats, according to roll-call data from Voteview.com, with only West Virginia Sen. Joe Manchin coming in to her right. Since joining the Senate, she’s taken public stances against Democratic efforts to abolish the filibuster and, along with Manchin, pushed her party to sharply reduce the outlays in budgetary legislation. Her positions have infuriated many Arizona Democrats, and the state party formally censured her over her 2022 vote to retain the filibuster, which helped block Democratic efforts to pass voting rights legislation.

Yet Sinema’s tendency to split from her party probably doesn’t explain her decision by itself. If that were the case, we might expect Manchin to have switched parties as well. In his study of party switchers from 1950 to 2014, political scientist Antoine Yoshinaka found that a member of Congress’s voting record didn’t do a good job of predicting party switchers on its own, even though party switchers were more likely to be out of sync with their party ideologically. He found that ideological disagreement usually had to occur alongside a desire to run for higher office or to gain a more valuable committee post to produce a greater likelihood of changing parties. Sinema, however, is already in a high office as senator — a presidential bid as a centrist independent seems unlikely, though we shouldn’t rule that out — and if she votes similarly to how she has previously, it won’t be as if she made a full-on party switch to the GOP and received a plum committee assignment as part of the deal.

But there’s a solid chance Sinema’s 2024 electoral outlook played into her decision-making process. Sinema hasn’t said whether she plans to run for reelection, but there’s little question that her tendency to break with her former party has outraged much of the Democratic base that helped put her in the Senate in the first place. A Suffolk University/Arizona Republic poll of the state in September found that Democratic likely voters viewed her quite negatively, with 49 percent holding an unfavorable opinion and 30 percent a favorable one. 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.

Still, 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. While Arizona does have 1.4 million registered voters who aren’t registered as Republicans (1.4 million voters) or Democrats (1.3 million), we also know that most self-identified independents lean toward one party or the other. So getting voters to eschew the major-party candidates won’t be easy, especially if Sinema isn’t overwhelmingly popular (the Suffolk poll put her overall favorability at 36 percent favorable, 38 percent unfavorable).

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, according to the U.S. Senate, as the table below shows:

Of this group, Sinema is the sixth to become an independent at some point, but only three prior to her actually stuck with identifying that way. Of those, Sen. Harry Byrd Jr. of Virginia might make the most interesting comparison to Sinema: A conservative Democrat, he refused to take an oath saying he would back the party’s nominee for president in 1972, ahead of his 1970 reelection campaign. He instead ran as an independent, thereby avoiding a primary challenge, and won a majority of the vote over Democratic and Republican opposition (he’d win in 1976 as well, although throughout that time he still caucused with the Democrats). Sinema, it seems, will be aiming to pull off the same trick in 2024 — that is, should she choose to run again.