Having time be against you is bad for negotiating.
Having another opportunity is good for negotiating.
So those two probably roughly balance out.
Just tell them you have another offer and ask whether it would be possible for them to move up the decision.
What are the alternatives? Keeping A waiting? Hoping you get an offer in time, but accepting A's offer if you don't? Those seem like significantly worse options than potentially hurting your negotiating position a bit (especially if you'd prefer to work for B regardless).