WV: Manchin isn’t dumb. I fully expect he will be able to tell that the race is unwinnable and simply retire (if that’s the case). He seems to know his state well. Safe R if he retires. I have to at least start off giving him the benefit of the doubt in 2024 since he has handled some very tough challenges, and a Democratic president gives him the opportunity to denounce whatever environment/insert left wing cause here legislation Biden might be supporting at the time like he did with Obama’s coal regulations.
MT: I don’t know enough about this state to say for sure.
OH starts out as a toss-up or Lean R. Republicans offered only token opposition with very little national support in 2018, and he still came close to losing.
Maybe he ran a lazy campaign, but a sitting four-term Congressman isn't exactly "token" opposition and a 7 point race isn't THAT close.
As happened this year, Democrats underperformed their polls in 2018, and this was particularly evident in Ohio. Throughout much of the year, it looked like Brown would win by double digits, but when he prevailed by the margin that he did, it became clear that he would have been locked in a tightly competitive race had he faced a more formidable candidate. And now, with Ohio drifting further to the right, as exemplified by Trump flipping both Lorain and Mahoning Counties, it's clear that Brown will be the underdog in 2024.
As for other Senate Democrats up in 2024, I agree with the consensus that West Virginia is Safe R. The state's last remaining officeholder aside from Manchin himself, Perdue, lost by 12%, and every other Democrat, including Jim Justice's undistinguished opponent, got wiped out. Shelley Moore Capito received more than 70% of the vote against Paula Jean Swearengin. Manchin knows that he has no shot at winning reelection, and he only barely scraped by with a 3% plurality against the odious Patrick Morrisey last time in a Democratic wave year. So he will retire, and that seat will flip. Tester is in for another brutal fight in Montana, but given that he's survived brutal fights three times now and is a formidable campaigner, I wouldn't rate his race as more than Tossup/Lean R.
Casey, Baldwin, Stabenow, and Klobuchar will probably all face more substantive challenges than they did in 2018. I think Klobuchar should be able to get by easily, assuming that she maintains her high approval ratings between now and then. I think the other three start as favorites as well, but they can't take things for granted. Rosen in Nevada and Sinema in Arizona will also be Republican targets, but they too, would probably start as favorites, and Sinema in particular, is a formidable campaigner with demonstrated crossover appeal. All other Democratic incumbents up that year are Safe to varying degrees.