The Senate battlefield is set after Tuesday’s primaries, with both parties bracing for a close — and expensive — fight for control centered on a handful of key states.

After GOP Sen. Rick Scott and Democratic former Rep. Debbie Mucarsel-Powell advanced to the general election in red-leaning Florida, the key November races on the Senate map are officially set, most of them in Democratic-held territory. 

Democrats are optimistic they can hold their narrow majority, buoyed by the fresh…