The decision by a senator from Arizona, Kyrsten Sinema, to leave the Democratic Party and register as an independent is the latest sign of weakness for the leader of the Senate Democrats, Charles Schumer.

Mr. Schumer won reelection in 2022 with about 55.3 percent of the votes in New York, according to the election night results from the state board of elections. That’s a decline from the 70 percent of the vote he drew in 2016, the 66 percent he attracted in 2010, and the 71 percent he garnered in 2004. 

The decline in voter support came even though Mr. Schumer outraised his Republican opponent, Joe Pinion, $41 million to $588,000, or nearly 70 to one, according to OpenSecrets.org.

Ms. Sinema’s December 9 Arizona Republic opinion piece explaining her decision doesn’t mention Mr. Schumer by name, but it reads like an attack on Schumerism. 

“Everyday Americans are increasingly left behind by national parties’ rigid partisanship, which has hardened in recent years. Pressures in both parties pull leaders to the edges, allowing the loudest, most extreme voices to determine their respective parties’ priorities and expecting the rest of us to fall in line,” she wrote. 

“In catering to the fringes, neither party has demonstrated much tolerance for diversity of thought. Bipartisan compromise is seen as a rarely acceptable last resort, rather than the best way to achieve lasting progress. Payback against the opposition party has replaced thoughtful legislating.”

She warned that legislative successes will become rarer “if party leaders stay more focused on energizing their bases than delivering for all Americans.”

She also accused party leaders of being “more focused on denying the opposition party a victory than they are on improving Americans’ lives.”  

Ms. Sinema said she’d continue her “opposition to tax hikes that would harm our economic competitiveness,” along with her effort to “secure the southern border.” Neither opposing tax hikes nor securing the border ranks high on Mr. Schumer’s to-do list. 

Mr. Schumer’s Democratic leadership team  for the new Congress includes far-left Senator Warren of Massachusetts as vice chairwoman of the conference and the socialist Vermont senator, Bernie Sanders, as “vice chair of outreach.” 

Mr. Schumer’s remarks on being elected as the Democratic leader mentioned “MAGA” — code for President Trump’s “make American great again” — nine times, which overpowered any mentions by Mr. Schumer of any sort of positive, substantive policy agenda.

Senate Democrats see themselves as having won a big victory by having avoided the fate of their House colleagues, who lost the majority to Republicans. Speaker Pelosi is no longer the Democratic leader in the House, but Mr. Schumer was re-elected by Democrats in the Senate as their leader. 

Spin aside, though, for Mr. Schumer, the reality is that after Sinema’s departure, even with the reelection of Senator Warnock of Georgia, the outcome falls far short of a triumph. It’s not even a roaring affirmation. The Democrats didn’t expand their Senate majority. The voters of New York didn’t exactly send Mr. Schumer back to Washington in a sedan chair.

Ms. Warren is up for reelection in Massachusetts in 2024, and Mr. Sanders is up for reelection in Vermont in 2024. Republicans nationally can look at Mr. Schumer’s decline and start working now to find and fund candidates capable of toppling Ms. Warren and Mr. Sanders. 

Even if they seem like long-shot battles, they are worth engaging. Mr. Schumer, Ms. Warren, and Mr. Sanders are precisely what Ms. Sinema is complaining about — “catering to the fringes,” “the loudest, most extreme voices.”

Democrats in the Senate spent the last two years fueling runaway inflation, salivating for a return to the Iran nuclear deal, dividing Americans along class- and race-based fault lines, failing to act on immigration or education choice while scheming to impose vast increases in taxes on income and capital. 

If they spend the next two years the same way, Mr. Schumer might wind up in the minority. In the meantime, who knows how many more voters and elected officials will follow Ms. Sinema’s example in declaring independence from Schumerism.