He doesn't need landslides. But solid wins. If he keeps winning,
he will continue to chip away. Look at the map, it's increasing Sanders territory.
You are right though that it's unlikely he will catch up in the end.
The Sanders campaign must really be kicking themselves. If they win the rust belt but lose the nomination because of the mega losses in the South it means they bungled it.
Vote your conscience.
I will offer you a different perspective though.
In this case "guilt by association" is relevant. Sanders is trying to point out that Hillary is always willing to sacrifice her principles to play politics, while he will hold true to progressive values and call for resignations even if politically unpopular.
He is trying to draw a contrast between principles and political calculation.
Being behind, he cant really afford to not "attack".
Hillary has also done the same back. Nra, Castro, etc.
The important thing is to vote.