Ensnaring Bridge

Oracle Text

Creatures with power greater than the number of cards in your hand can't attack.

Card Rulings

10/4/2004 All creatures on the battlefield (including yours) are subject to the restriction based on the number of cards in your hand.
10/4/2004 It only checks the cards in your hand when declaring attackers. Once they are declared, changes to your hand size or to the power of the creatures will not remove creatures from the attack.