Like any vegetable, broccoli raab will keep longer in the refrigerator unwashed until you're ready to use it.

Stored properly, broccoli raab will keep for about a week in a plastic bag. If your broccoli raab is damp, whether from the farm or store or because you've already washed it, you can line the plastic bag with a clean kitchen towel or paper towel to absorb condensation. 

It's also a good idea to remove any ties holding the stems together to help moisture wick away.