Andrew Prokop explains “Why Republicans are favored to win the Senate — and how Democrats could stop them” at Vox:

In the past decade, there have been 20 individual Senate elections where a seat ended up flipping to the other party. The vast majority of those races (16 of 20) had the same partisan outcome as either the presidential race that year or, in midterm years without a presidential contest, the most recent one. Senate races have been falling in line with the state’s presidential party preference. “Mismatched” senators, who represent a state their party’s presidential nominee lost, are becoming rarer.

From that perspective, Democrats have a pretty okay map in 2022. In the two most recent midterm cycles, they were badly exposed, with several incumbents in states the Republican presidential candidate just won. This year, they have none at all. (They do have three such seats coming up in 2024, which will be a major challenge, but that’s a problem for another time.) Meanwhile, there are two GOP-held seats in states Biden narrowly won, Pennsylvania and Wisconsin, on the ballot.

But that’s likely too optimistic for Democrats. Another way to think about the map is that there are six true swing states with races this cycle. At least once in either 2016 or 2020, Trump either won or came quite close to winning Pennsylvania, Wisconsin, Georgia, Arizona, and New Hampshire. Nevada, meanwhile, trended right relative to the country between 2016 and 2020, though Biden still won it….Senate seats in these states are in great danger of slipping out of Democrats’ grasp.

But while Senate race outcomes have become more correlated with national partisanship, individual candidates do frequently overperform or underperform the overall trend. Democrats’ Senate chances likely hinge on whether enough of their candidates can escape this partisan gravity, arguing either that they’re not just another Democrat, or that their opponent is a uniquely unfit Republican.

Prokop shares some indepth analysis of races in the six states, and provides this chart to illustrate Democratic hopes for picking up Senate seats:

History, as well as current polls, indicate that Democrats are facing an unusually tough combination of Biden’s lagging approvals numbers and a deteriorating party image. As Prokop writes, Democratic hopes require that “unique dynamics among candidates in individual races break in their favor.”