In “Why November could decide Senate control for years,” Ronald Brownstein writes at CNN Politics:

In this fall’s Senate elections, Democrats will be defending more seats in precarious political terrain than in any other election during the 2020s. That list of challenging elections this year includes the final three Senate seats Democrats hold in states that voted for Donald Trump in 2020, and five more in states that President Joe Biden won by 3 percentage points or less. Meanwhile, Republicans this year are not defending any Senate seats in states that voted against Trump in 2020, or preferred him by 3 points or less.

That math underlines the stakes for Democrats in Biden improving his position in the key swing states by November. One of the most powerful trends of modern Senate elections is that it has become exceedingly difficult for candidates in either party to win seats in states that usually vote the other way for president.

The Senate Democrats running in difficult electoral terrain might break that trend this fall. Yet if they can’t, Biden’s fate in November could determine control of the Senate not only in 2025, but for years thereafter.

A strong recovery by Biden in which he wins most of the key swing states could position Democrats to remain competitive in the battle for Senate control through the remainder of this decade, even if they narrowly lose the majority in November. But if Biden loses most of the swing states, Democrats could fall into a Senate deficit too large to realistically overcome for years — especially because the party has so few plausible opportunities to flip seats now held by the GOP.

Brownstein adds that “That prospect has enormous implications not only for the passage of legislation but also for the composition of the federal courts, especially the Supreme Court. Four of the Supreme Court justices will be older than 70 by 2028. Even if Biden holds the White House in 2024, and a vacancy arises, a durable Republican Senate majority might refuse to fill any of those seats — just as then-Majority Leader Mitch McConnell did while Barack Obama was president in 2016.”

Further, Brownstein notes,”Heading into the 2024 election, Republicans hold 47 of the 50 Senate seats in the 25 states that voted for Trump in 2020. Democrats, in turn, hold 48 of the 50 Senate seats in the 25 states that voted for Biden last time….In 2020, Biden won three states by less than a single percentage point: Arizona, Georgia and Wisconsin. He won three others by less than 3 percentage points: Pennsylvania, Nevada and Michigan. Democrats now hold 11 of the 12 Senate seats from those six highly competitive states.“

“By contrast,” Brownstein reports, “among the 25 states that backed Trump, North Carolina — where Republicans hold both Senate seats — was the only one Trump carried by less than 3 percentage points. Even extending the net to states Trump won by less than 5 percentage points brings in only Florida, where Republicans also hold both Senate seats….This contrast creates a huge disparity between the parties. Democrats now hold 14 inherently vulnerable Senate seats: their three from the states Trump won in 2020, and their 11 in the states Biden won only narrowly. For Republicans the total is at most six: two in states that Biden won in 2020, and four in states that Trump won narrowly, even with Florida included.” Also,

This year’s Senate races in the narrow Biden states include Democratic incumbents Bob Casey in Pennsylvania, Tammy Baldwin in Wisconsin, and Jacky Rosen in Nevada; also on the ballot is an open Democratic-held seat in Michigan (where the party is very likely to nominate Rep. Elissa Slotkin) and the Arizona seat held by Kyrsten Sinema, an independent who caucuses with Democrats. Sinema hasn’t indicated whether she will seek reelection, but Democratic Rep. Ruben Gallego is already running for the seat. (Democrats also face an unexpected challenge in Maryland — a state that leans much more reliably toward them in presidential elections — after GOP former Gov. Larry Hogan last week said he would run for the open seat there.)

In addition, Democrats this year must defend all three of their remaining Senate seats in the states that voted for Trump in 2020. That includes incumbents Jon Tester in Montana, Sherrod Brown in Ohio, and the open seat being vacated by the retiring Joe Manchin in West Virginia. Neither of the two remaining Senate Republicans in states that Biden won last time (Susan Collins in Maine and Johnson in Wisconsin) are up this year; nor are either of the GOP senators from North Carolina, the state Trump won by his smallest margin.

Both parties agree the open West Virginia seat is virtually guaranteed to flip to the GOP. Tester and Brown both have strong personal brands, but Biden is almost certain to lose their states, and possibly by substantial margins. If he does, Brown and Tester could survive only by breaking a nearly inviolate recent pattern in presidential election years.

“If Democrats lose Senate seats in the narrow Biden states,” Brownstein points out, “they simply have very few places on the map to replace them, given the parties’ patterns of support. It’s that prospect that has led the Democratic data analyst David Shor to warn for years that if the party doesn’t perform well in the 2024 presidential election, the GOP could seize control of the Senate for a sustained period.”

There are some more optimistic scenarios for Democrats, as Brownstein explains: “David Bergstein, communications director for the Democratic Senatorial Campaign Committee, notes that in the 2022 midterm election, Democrats won Senate races in most of these same battleground states, although exit polls showed substantial discontent with the economy and Biden’s performance then, too. “Senate campaigns are candidate versus candidate battles,” Bergstein said. “We have the superior candidates, and Republicans are putting forward candidates who have big flaws, who lost races before, or are facing the prospect of damaging primaries. Certainly a presidential cycle is different than a midterm, but the laws of Senate campaigns, where candidate quality matters, are still in effect.”

Brownstein concludes, “Unless and until such a new political configuration emerges, both parties can realistically target many fewer Senate seats than they could even two decades ago. But the ceiling is clearly lower for Democrats than for Republicans. It leaves Democrats, even in good years, with achingly little margin for error to build a Senate majority. And unless Biden recovers more strength, 2024 may be very far from anything Democrats would call a good year.”