Make the Margarita Base: Combine all ingredients (except ice) and blend in blender until smooth. Transfer to a large glass measuring cup. Can be made ahead. Store in refrigerator.
To mix the final drink: Pour half of the margarita base into blender. Add 3 1/2 cups ice cubes. Blend until mixture is slushy; pour into salt-rimmer margarita glasses. Garnish with cranberries. Repeat with remaining margarita base and remaining ice.