If you want to win and you are not already the leader, you have to go after the leader, not the person in third or fourth place.
If you are holding back on attacking Sanders because you think you’ll need his supporters later, please note that you will never get his supporters if you continue to let him lead, no matter how gentle you are to him. While he is in the race, his supporters are his.
If you hold Sanders to plurality wins, you will never get the lead. And you will never get his supporters.
If you don’t want to go after Sanders, you should drop out immediately because all you’re doing is splitting the non-Sanders vote – for example 2016.