At this point, a Democrat in a close race would be savaged by the dem base if he broke ranks. A Democrat who is a couple of years away from a reelection might benefit, but it would be suicidal at this point if he’s up this November.
It might be safe to say that Donnelly is opposing Kavanaugh simply because he’s auditioning for his next job.
A case of damned if you do and damned if you don’t for Democrat Senators in states that Trump carried big.