Thirty three Senate seats will be up in November, with Democrats having to defend 23 seats and Republicans only 10 seats, so a major struggle for Democrats to keep control of the Senate, which right now is divided 51-49 with three Independents helping the Democrats keep the majority.
The major battles to take Republican seats will be in Texas (Ted Cruz); Florida (Rick Scott); and Missouri (Josh Hawley).
The most dangerous Democratic seats are Arizona, Maryland, Montana, Nevada, Ohio, Pennsylvania, West Virginia, and Wisconsin.
It would seem certain that West Virginia, with the retirement of Joe Manchin, will go to the Republicans, and Maryland, normally a strong blue state, has the problem of popular former Governor Larry Hogan running for the Senate, which makes it seem likely that the state will flip to the Republicans.
Ohio with Sherrod Brown, and Montanea, with Jon Tester, are also in danger, while Arizona, Nevada, Pennsylvania, and Wisconsin more likely to remain Democratic.
So assuming West Virginia and Maryland go Republican, that would mean a 51-49 Republican controlled Senate, unless the Democrats can win at least one seat from the three Republicans who are contestable–Texas, Florida, and Missouri.
And this is assuming Joe Biden wins a second term, and Kamala Harris is there to break a possible 50-50 tie in the Senate.