Meanwhile, heat the olive oil in a large pot over medium heat. Add the garlic and broccoli rabe, season with salt and pepper and cook until the broccoli rabe is slightly tender, about 6 minutes. Transfer to a plate. Add the sausage to the pot and cook until just brown, breaking it up with a spoon, about 5 minutes. Add the tomatoes and cook until soft, about 2 minutes. Add the wine, bring to a boil and cook 3 minutes.
Add the clams; cover and cook over medium-high heat until the clams open, 5 to 7 minutes (discard any that do not open). Return the broccoli rabe to the pot and season with salt and pepper. Divide the polenta among bowls and top with the sausage-clam mixture and cooking liquid. Sprinkle with parsley, if desired.