Sen. Dick Durbin's (D-Ill.) retirement is setting off a once-in-a-decade leadership fight for Senate Democrats.

Why it matters: Durbin, 80, has been the Senate Democratic whip since 2005. Whoever replaces him becomes the instant front-runner to replace Senate Democratic leader Chuck Schumer down the road.

• An open fight for the whip spot next year could also complicate Schumer's strategy of privately mediating intraparty disputes.
• It will potentially expose the party's deep divide on whether to elect a moderate — or a progressive — to help them return to power.

Zoom in: Sen. Brian Schatz, 52, of Hawaii is actively making outreach to members about replacing Durbin. His interest in the job is a poorly kept secret on Capitol Hill, and he's got a head start as the current chief deputy whip.

• Sen. Amy Klobuchar, 64, of Minnesota wants the job, sources tell us. Schumer made her the No. 3 Senate Dem last year, and she's respected by her colleagues.
• Sen. Catherine Cortez Masto, 61, of Nevada is also interested in moving up, although it's not clear if she's ready to challenge Schatz or Klobuchar.

Between the lines: Schatz and Cortez Masto voted with Schumer last month in support of a GOP spending plan to avert a government shutdown.

• Those votes were seen as key indicators of their interest in leadership gigs.
• Klobuchar voted against the measure, as did Sen. Cory Booker (D-N.J.), another one of Schumer's top leadership officials this year.
• Schumer intervened this year when Klobuchar and Booker were on a crash course for the leadership spot vacated by former Sen. Debbie Stabenow (D-Mich.).

Zoom out: Durbin's retirement has also ignited what could be a bitter Illinois primary battle to succeed him in the Senate, we reported today.

• Durbin predicted "at least a dozen" candidates will jump into the race.
• Reps. Lauren Underwood and Raja Krishnamoorthi could start out as the front-runners in the big-dollar race, according to polling data released last month.
• Krishnamoorthi has a $19 million war chest, and Underwood has over $1 million in cash on hand. Rep. Robin Kelly, another possible candidate, has $2 million in cash.

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— Stephen Neukam, Hans Nichols and Andrew Solender

Schumer is setting expectations pretty high for a party that's still feeling pretty low.

• "We are going to be in the majority in 2027, " Schumer told Semafor's Burgess Everett.
• "The electorate will desert Republican candidates who embrace Trump, in an overwhelming way."

Between the lines: By promising the majority, Schumer has now done what all leaders of the minority do when they're not in power.

• But the risk for Schumer is that he is promising something that few of his colleagues believe he can deliver.
• But there would be an even greater downside for him — disengaged donors, despondent activists — if he accepted minority status for more than two years.

— Hans Nichols

Durbin's retirement also means the Senate Judiciary Committee will have a whole new look next year.

Why it matters: Sen. Chuck Grassley (R-Iowa), the current chair, is term-limited after this Congress and will relinquish the top spot in 2027.

• He and Durbin have a combined 72 years in the Senate. Along the way, they learned to work together.
• Grassley, 91, wrote on X about Durbin: "sorry 2see him retire from the Senate Obviously he's liberal & I'm conservative yet we hv a wonderful working relationship."

What we're watching: The next chair and ranking member of the committee will play a key role in vetting Trump's judicial nominees.

• But if a Supreme Court justice decides to retire before the midterm election, the two senators could have their last joint act on the committee vetting the nominee.

— Hans Nichols and Stef Kight