It is an open seat in a state that is generally Republican. It's house, sneate, and governor are Republicans, and Florida usually goes GOP in Presidential elections. It is a seat that should have been picked up.
You're assuming Rep. Harris would have won the Senate seat. That's a strong assumption.
Harris did have a 5 point lead over the Democrat, the GOP leading candidate is now trailing by about 5. So far it does not appear to be a brilliant move. It was a very winnable seat, but the Dems appear to have the advantage now.
FL's Senate seats are both Dem. The state is about evenly divided, and a race polling closely with Harris is still polling closely without her.