Sen. Kyrsten Sinema (I-AZ) defecting from the Democrat Party to become an “Arizona Independent” creates a big headache for her old party as it tries to keep its slim 51-seat majority in the United States Senate in 2024.

In the next election cycle, 23 of the 33 Senate seats up for reelection are currently held by Democrats or left-leaning Independents, and former President Donald Trump won six of the states by double digits in at least one of his presidential elections.

The New York Times acknowledged that some of the hardest seats for the Democrats to keep will be Sen. Jon Tester’s (D) Montana seat, Sen. Sherrod Brown’s (D) Ohio seat, and Sen. Joe Manchin’s (D) West Virginia seat. Trump won those states by 16, 8, and 29 percent in 2020.

But, Sinema’s party hop puts another damper on the Democrats looking to minimize losses in 2024.

Sinema left the party last week by announcing she had registered as an “Arizona Independent” during an interview last Friday. She acknowledged that “a growing number of Arizonans and people like me just don’t feel like we fit neatly into one party’s box or the other.”

However, even before her announcement, Democrats had already talked about having someone primary her when she’s up for reelection in 2024 for not being far-left enough. Rep. Ruben Gallego (D-AZ) has even been floated as a potential primary candidate against her.

On Wednesday, Politico explained one option that would allow the Arizona senator’s party switch not to cause a contested Democrat primary but could end up making a “nightmare scenario” for the party:

With three candidates on the ballot, a GOP nominee can capitalize on centrist and liberal divisions and win a Senate seat with a plurality vote. Democrats are not eager to intervene at the moment, but at some point they may have to make a call about whether to support Sinema, back whoever wins a primary or sit out the race altogether.

So, with only a slim 51-seat majority, the Democrat Senatorial Campaign Committee (DSCC), the Senate Democrats’ campaign arm, will not only have to try to keep the 51-seat majority but will also potentially have to spend millions of dollars protecting Sinema, a valuable incumbent for the left, in addition to some Democrat senators running in states that have turned redder over the years.

Jacob Bliss is a reporter for Breitbart News. Write to him at jbliss@breitbart.com or follow him on Twitter @JacobMBliss.