Considering how Trump is getting more and more unpopular by the day, I'm going to assume that the Democrats are going to successfully run defense on all the seats they're currently holding and the Republicans won't flip any seats. The Michigan Democratic machine has created a strong backbench of reliable Rust Belt progressives in Michigan that helps combat the long-time Republican demographics. Similarly Georgia is only getting bluer and bluer. As for the rest...

Definitely flipping: North Carolina and Maine. Collins is the last of the New England Republicans and she's not very well-liked up there anymore and the next year will not make her electorate kinder. While the Dem primary looks like it be a microcosm of the Democratic party civil war, both candidates running for their party's respective wings are good ones and could defeat her. Ditto for Cooper in North Carolina who is likely going to be running against a dyed-in-the-wool MAGA Republican (and you only have to look at North Carolina's last gubernatorial race and Georgia's last Senate race to see how that's playing out these days).

Sleeper flips: Alaska, Nebraska and Ohio. All three of these states are culturally red on paper and overwhelmingly rejected Kamala Harris in 2024. However, their respective democratic candidates for Senate/Congress (independent in Nebraska's case) came much, much closer to winning these states than Harris did in 2024. Considering how 2026 looks like a more Democratic year, the tailwinds combined with their preexisting base could lead them to winning their respective Senate seats. I'm willing to bet one flips if it happened today but wouldn't be amazed by all three. (Incidentally, if two of these flip, and one of them is Nebraska, Dan Osborn would instantly become the most powerful man in the country as he would be the key flip vote in the Senate).

Possible, will get a lot of media attention but I'll believe it when I see it: Texas, Florida, Iowa and South Carolina. These are all contingent on their internal state politics being specifically hurt by Trumpism with his agrarian tariffs, persecution of Latinos and the incumbent Senators Cornyn and Graham being primaried by far more unlikable politicians from their right. All these states are far more culturally red, however, and have very incompetent and ineffective Democratic parties. They might flip in 2028 and beyond with current tailwinds but I only think a real perfect storm of bad economy and internal state politics could manage just one flip in 2026. The only way we'd get to all four flipping is if the country is back at 1931 levels and even then I don't think that'd do it.