A potential mess in Arizona was an unwelcome surprise for Democrats while they were still savoring their victories in 2022.When Senator Kyrsten Sinema left the Democratic Party last week, she didn’t just momentarily drive up antacid sales on Capitol Hill. She also raised the pressure on three especially vulnerable Democratic senators who are up for re-election in 2024, and are defending seats in states that have turned a shade of deep crimson since they were first elected to Congress.The 2024 map is daunting for Senate Democrats, and it will take all the political dexterity and luck they can muster to keep their 51-ish-seat majority — and then some. Twenty-three of the 33 seats up for grabs are held by Democrats or left-leaning independents. That list includes Montana, Ohio and West Virginia, where Donald Trump won in 2020 by 16, 8 and 29 percentage points.But daunting is not the same thing as impossible. Faced with steep odds in the past, Democrats have managed to find local causes to champion — remember Senator Tammy Baldwin of Wisconsin’s crusade against almond milk? — as they looked for ways to differentiate themselves from the national party. And their incumbents have proved doubters wrong in the past.“From 30,000 feet, it looks brutal, but as you get closer to the ground, I feel more optimistic about it,” said Jim Kessler, vice president for policy at Third Way, a center-left think tank. “If it’s mainstream versus extreme, we have a great shot.”For now, Democratic strategists are still poring over the results of the recent midterm elections, trying to gain a deeper understanding of what moved voters.One consensus viewpoint so far, at least among those I’ve spoken with: Democratic candidates earned just enough credit for trying to address inflation through moves like capping insulin prices to dull Republicans’ advantage on the economy. And they say that while abortion may not matter quite as much in the next election, the issue is not going away in 2024.Another lesson is crystal clear: Trump has become even more toxic to swing voters during his two years in exile. The latest evidence? A USA Today/Suffolk University poll shows Trump losing a hypothetical matchup with President Biden by nearly eight points.On the other hand, there are no signs that any of these three states have grown less difficult for Democrats over the last six years. It’s easy to forget that Barack Obama won Ohio twice, or that Montana had a Democratic governor as recently as 2021. Today, that feels like ancient history.Once Democrats turn to 2024 in earnest, their first and most important task will be ensuring that their incumbents run again. As for Republicans, they are still debating what went wrong this year, with much of the discussion centering on the mechanics of campaigns, like mail voting and ballot harvesting — rather than thornier issues, like abortion. At the same time, as G.O.P. candidates begin declaring their intentions, many are still treading cautiously when it comes to Trump.“Some of the primary noise on their side suggests they haven’t learned too much yet,” said J.B. Poersch, the president of Senate Majority PAC, a group closely associated with Senator Chuck Schumer. “There’s plenty of things for them to be nervous about.”The Democrats’ red-state defendersSo far, of the Democratic incumbents in those three states above, only Senator Sherrod Brown of Ohio has definitively said he’s in. Brown has demonstrated a unique knack for winning working-class voters, even as cultural factors start to outweigh economics. He won his race by nearly seven points in 2018, while Representative Tim Ryan lost to J.D. Vance this year by roughly the same margin — far less than other statewide candidates in Ohio, but hardly encouraging for Democrats.Republicans are lining up to take on Brown, notably State Senator Matt Dolan, who finished third in this year’s Senate primary behind Vance and Josh Mandel — both of whom aggressively courted Trump and his base.Dolan, whose family owns the Cleveland Guardians, is already exploring the boundaries of what constitutes acceptable criticism of Trump. “What we witnessed nationally should convince us the country is ready for substantive candidates, not personalities and election deniers,” he wrote in a recent email to Republican county chairs in Ohio. But he said he would support Trump if he were the nominee.Then there’s Senator Joe Manchin of West Virginia, who sounds intrigued by Sinema’s decision to become an independent. “I don’t know how you get more independent than I am,” Manchin told reporters at the Capitol on Monday. “I look at all of these things, I’ve always looked at all of these things. But I have no intention of doing anything right now.”Like most things Manchin, that answer was neither a yes nor a no. He added, “I’m not a Washington Democrat.”Manchin already has an official Republican challenger: Representative Alex Mooney, who has telegraphed his line of attack in an anti-Manchin ad that ran four months ago. At least two others have shown interest: Attorney General Patrick Morrisey, who ran against Manchin in 2018, and Gov. Jim Justice, who is term-limited.Montana is only slightly less intimidating terrain for Democrats. They lost both House races this year, while Republicans won a supermajority in the State Legislature.Senator Jon Tester of Montana is skilled at finding locally resonant issues to champion.Sarahbeth Maney/The New York TimesSenator Jon Tester has said he will make a decision about running again after the holidays, though he has told reporters he feels “very positively about my chances.” Tester, who heads home to his farm most weekends, is skilled at finding locally resonant issues to champion, such as federal support for rural hospitals or floodplain mapping.Tester allies point to an emerging dynamic on the Republican side that resembles what happened in many primaries in 2022: a race to the right.One possible contender is Representative Matt Rosendale, whom Tester defeated in 2018 and who is staking out a position as one of the holdouts to Representative Kevin McCarthy’s bid to become House speaker. Another is Representative Ryan Zinke, who resigned as Trump’s interior secretary amid a flurry of investigations into his conduct. He will return to Congress early next year after winning by just three points against Monica Tranel, a political novice, despite outspending her by two to one.The rest of the mapAt the moment, Democrats appear to have just two pickup opportunities, and neither looks especially promising: Florida and Texas.And even the seemingly more comfortable seats they hold, like Nevada and Pennsylvania, are not all that comfortable. Nevada was the closest of all the big Senate races this year, with Senator Catherine Cortez Masto winning by fewer than 8,000 votes.In Pennsylvania, Republicans are hoping that David McCormick, who lost narrowly to Dr. Mehmet Oz in the primary this year, will challenge Senator Bob Casey in 2024. Democrats saw McCormick, a former hedge fund executive with deep pockets and roots in Pittsburgh, as the more formidable potential opponent, and subtly tried to help Oz. McCormick is planning to release a book in March, “Superpower in Peril: A Battle Plan to Renew America,” that appears aimed at positioning him more squarely as a China hawk, shoring up a point of vulnerability that hurt him this year.“I’d be shocked at this point if he doesn’t run,” said Josh Novotney, a former aide to Senator Pat Toomey and a partner at SBL Strategies, a lobbying firm in Pennsylvania. But Novotney cautioned that if Trump were the nominee, it could doom Republicans’ chances of defeating Casey. In the 2022 Senate race, Oz was weighed down by Trump and by Doug Mastriano, the Republican nominee for governor, whose hard-line stances on abortion and embrace of election denialism repelled swing voters.Democratic senators are also up for re-election in Michigan and Wisconsin, where their chances look brighter. In 2018, Baldwin crushed her Republican opponent, Leah Vukmir, by nearly 11 points, while in Michigan, Senator Debbie Stabenow cruised to victory over John James, who opted to run for a House seat rather than face Stabenow again. This year, Gov. Gretchen Whitmer won re-election easily, while Democrats took full control of the State Legislature for the first time in 40 years.Sinema’s defection undeniably makes Democrats’ path more complicated. She has not said she is running, though many political observers suspect her decision to switch parties had to do with worries she would lose a Democratic primary. Neither of the two most prominent Democrats weighing a run, Representatives Ruben Gallego and Greg Stanton, has officially entered the race, however.Republicans in Arizona could nominate someone on the far right, such as Sheriff Mark Lamb, or a moderate like Karrin Taylor Robson, a lawyer who lost to Kari Lake in this year’s primary for governor. So although most analysts assume that a three-way race would help Republicans, there are too many variables to draw any firm conclusions — including whether there will even be a three-way race.For now, Democrats are philosophical about the 2024 landscape. “Every election,” Poersch said, “you’re testing: Have the rules changed, or are we playing by the same old rules?”What to readDespite modest improvements for Republicans in 2022, Democrats largely held onto their gains among suburban voters, particularly in battleground states, Trip Gabriel reports.Donald Trump’s family business lost a criminal contempt trial that was held in secret last fall, according to a newly unsealed court document and several people with knowledge of the matter. Jonah Bromwich, William Rashbaum and Ben Protess explain.President Biden signed a bill mandating federal recognition for same-sex marriages and capped his evolution toward embracing gay rights over a four-decade political career. Michael D. Shear has the details.Inflation slowed more sharply than expected in November, Jeanna Smialek reports. It was an encouraging sign for both Federal Reserve officials and consumers and raised hopes for a “soft landing,” or one in which the economy slows gradually and without a painful recession.Thank you for reading On Politics, and for being a subscriber to The New York Times. — BlakeRead past editions of the newsletter here.If you’re enjoying what you’re reading, please consider recommending it to others. They can sign up here. Browse all of our subscriber-only newsletters here.Have feedback? Ideas for coverage? We’d love to hear from you. Email us at onpolitics@nytimes.com. More