Democrats currently control 53 seats in the Senate (including two independents who caucus with the Democrats), Republicans 47.
Here's how we define our ratings categories:
Safe: Barring unforeseeable developments, one party is certain to win.
Race to Watch: A foreseeable but as-yet unrealized development has the chance to make an otherwise "Safe" race potentially competitive, or an incumbent faces a potentially competitive primary.
Likely: One party has a strong advantage and is likely to win, though the race has the potential to become more competitive.
Lean: One party has an identifiable advantage, but an upset victory is possible for the other party.
Tossup: Both (or all) parties have a strong (though not necessarily perfectly equal) chance of winning.