"I think Elizabeth Warren and Hillary Clinton will be sniping at each other by spring," predicts Valliere.
I looked them up here but still couldn't understand them.
Did you get the impression that by spring is an idiom or fixed expression? Actually it's just the words by and spring. So we have three words here that are confusing you: sniping (present participle of the verb to snipe), by and spring.
To snipe means:
And spring is: