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.
You can motivate this by saying you'd prefer to work for them, which also shouldn't hurt your negotiating position, because most employers would much rather want an employee that actually wants to work for them as opposed to one who will just jump ship once they get a better offer.
What are the alternatives? Keeping A waiting? Hoping you get an offer in time, but accepting A's offer if you don't? Declining A's offer and potentially getting neither? Those seem like significantly worse options than potentially hurting your negotiating position a bit (especially if you'd prefer to work for B regardless).