Sen. Kyrsten Sinema's (I-Ariz.) decision not to seek re-election has dealt the latest in a series of crushing blows to Senate bipartisanship, hollowing out a centrist core that has suffered under years of intensifying polarization.

Why it matters: The departures of Sinema, Sen. Joe Manchin (D-W.Va.) and Sen. Mitt Romney (R-Utah) — three moderates routinely vilified by their own parties — will leave a massive hole for bipartisan deal-making.

• It's hard to look at today's political incentives, with lawmakers more concerned about their primaries than the general election, and conclude that the vacuum will be filled.

Zoom in: Despite her broad unpopularity, Sinema will leave Congress with a virtually unparalleled record as a bipartisan negotiator.

• The freshman senator wielded outsized power as a moderate swing vote, helping to craft bipartisan laws on infrastructure, gun safety, marriage equality and semiconductor manufacturing.
• Sinema was the first Democrat to win an Arizona Senate seat in decades, but she left the party in 2022 — a political earthquake that came just three days after Democrats secured a 51-49 Senate majority.

The other side: Some progressives think of Sinema as a sellout. She helped advance some of President Biden's top priorities — but when the political road got bumpy, the competitive triathlete wouldn't go the distance.

• Her and Manchin's staunch opposition to eliminating the filibuster, for example, prevented Democrats from passing landmark voting rights legislation in 2022.
• Sinema also stood in the way of Biden's signature spending package — the Inflation Reduction Act — until Democrats abandoned a tax provision that would have closed the "carried interest loophole" for private equity.

Zoom out: Sinema, Manchin and Romney have spent years sounding the alarm on the death of bipartisanship, a trend punctuated by the GOP's recent rejection of the border security deal that Sinema helped negotiate.

• "You lose the center, you lose the moderates, you're screwed. You really are screwed," Manchin told Politico after Romney's retirement announcement. "I'm hoping the voters will wake up."
• Even the exit as Senate GOP leader of Sen. Mitch McConnell (R-Ky.) — once viewed as the quintessential partisan brawler — was lamented by Democrats who admired his commitment to the institution.

Between the lines: Of the 10 senators who helped negotiate the bipartisan infrastructure law in 2021, four will be gone by next year.

• A fifth — Sen. Jon Tester (D-Mont.) — faces a highly competitive re-election race in November that could determine control of the Senate.
• Sens. Susan Collins (R-Maine) and Lisa Murkowski (R-Alaska) remain pillars of the Senate's centrist foundation, but there's little doubt they're endangered species.