It might be too soon to report all that the resignation from the Democratic Party of Kyrsten Sinema means for the coming battles in the 118th Senate. It’s not too soon, though, to grasp that it’s an important moment. The savvy Arizona solon says she’s not going to caucus with the GOP. Neither, though, does she say she’ll caucus with the Democrats. It does deny the Democrats an absolute majority in the upper chamber.

Ms. Sinema’s announcement comes at the end of a hard-fought midterm election cycle that ended with Democrats faring better than they might have feared, considering President Biden’s low approval ratings and inflation raging at levels not seen in four decades. Republicans only barely captured control of the House, and the Democrats ended up improving their position in the Senate after Tuesday’s runoff race in Georgia.

While Ms. Sinema’s decision may not fully impair the Democrats’ control in the Senate (she will do her Committee assignments via the Democrats), it might pop the “exuberant” feeling to which Senator Schumer professed following the re-election of Senator Warnock, which appeared to hand him an actual majority in the upper chamber. Mr. Schumer permitted himself a victory lap of sorts, in remarks to reporters after the results were known.

“This election” shows that “Americans believe in democracy,” Mr. Schumer crooned after the Georgia runoff. They “rejected so many of this MAGA extremism (sic) — both in knocking down and not voting for extreme MAGA candidates.” Mr. Schumer said he could breathe a “sigh of relief,” ABC News reported, because “Democrats have total control of the Senate” and would no longer need a “power-sharing agreement with Republicans.”

Mr. Schumer might have been just a bit ahead of his skis. Ms. Sinema made her announcement in an op-ed piece in the Arizona Republic in which she characterized her move to independence as a “natural extension” of her service since she was first elected to Congress. No doubt that she has had an independent streak, which we and a lot of others who favored the principles of the Republican Party admired from a distance — and shared.

This newspaper, incidentally, followed a similar transition during the 19th century, starting out more with the Democrats. After the Civil War, though, the Sun was purchased by one of Lincoln’s allies, Charles Dana. It admired General Grant, then emerged late in the century, as the leading backer in the of conservative principles — limited and constitutional government, sound money, equality under the law, and against government largess.

Ms. Sinema’s move could be seen as an echo of President Reagan’s quip over his change in party affiliation: “I didn’t leave the Democratic party, the Democratic Party left me.” Described by NPR as “unapologetically moderate,” Ms. Sinema was the target of criticism from her own party for her failure, along with Senator Manchin, to endorse the party’s most liberal agenda items in Congress — including the abandonment of the Senate filibuster.

Arizona Democrats censured her for a “failure to do whatever it takes to ensure the health of our democracy.” Congresswoman Alexandria Ocasio-Cortez excoriated her for refusing to truckle to the far left and at one point of “tanking your own party’s investment on childcare, climate action, and infrastructure” and even “choosing to exclude members of color from negotiations.” Ms. Sinema was even chased into a bathroom by a leftist mob.

Ms. Sinema explains in the Republic that she sought to join the Senate “to be independent and work with anyone to achieve lasting results.” She reiterated her commitment to civility: “I would not demonize people I disagreed with, engage in name-calling, or get distracted by political drama.” After the treatment she has received from the members of her own party, it’s no wonder Ms. Sinema is seeking to chart a new course.

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This editorial has been updated following the bulldog.