If a creature's power is reduced to 0 by a continuous effect, can it still deal combat damage?

A creature deals combat damage equal to its power at the time damage is assigned and dealt. If a continuous effect (such as a pump spell wearing off, or an opponent's debuff like Diminish) reduces a creature's power to 0, that creature assigns exactly 0 combat damage to whatever it is attacking or blocking. Zero damage is still technically 'dealt,' but 0 damage has no practical effect — it won't cause loss of life, destroy creatures via lethal damage, or trigger most damage-based abilities.

The key rules are CR 510.1a (attacking creatures assign combat damage equal to their power) and CR 510.1b (blocking creatures assign combat damage equal to their power). Both rules anchor the damage amount directly to the creature's current power, so a power of 0 means 0 damage assigned.

It is important to note that the power is checked at the time damage is actually dealt (CR 510.2), not when blockers are declared. So if the power reduction happens after blockers are declared but before the Combat Damage step resolves, the creature will still deal 0 damage based on its power at that moment.

Concrete example: Your 2/2 Bear attacks. Your opponent casts Dead Weight (−2/−2 enchantment) on it during the Declare Attackers step. When the Combat Damage step arrives, the Bear is now a 0/0 — wait, actually it would die from 0 toughness before damage. Adjust: suppose it's a 3/3 Bear and Dead Weight gives −2/−2, making it 1/1 — it still deals 1 damage. But if instead your opponent uses an effect making it 0 power (e.g., a 3/3 reduced by −3 to power only, staying alive), it deals exactly 0 combat damage.

In summary, a 0-power creature cannot deal meaningful combat damage. It participates in combat normally (can block, can attack, can be blocked) but contributes nothing in the damage step. This is governed by CR 510.1a, 510.1b, and 510.2.