to be the person or group that has the most power in a situation.
So long as we rely on them for money, they have the whip hand.
hold the whip hand
To have the most power or influence in a particular situation or setting. As the editor in chief, I hold the whip hand and give final approval for the layout of every issue—remember that.
get, have, hold, etc. the ˈwhip hand (over somebody)
have power or control (over somebody): The government knows that the army have the whip hand.Our opponents had the whip hand over us right from the beginning.