Does damage dealt during the combat damage step use the creature's power at that moment or when it was assigned?

When the combat damage step begins, the active player assigns combat damage to targets first, then the non-active player does the same. Under CR 510.1, each attacking and blocking creature assigns combat damage equal to its power at the time damage is assigned. This assignment happens all at once before any damage actually resolves.

After assignment but before damage is dealt, players do get priority and may cast instants or activate abilities (CR 510.2). However, because damage amounts were already locked in during the assignment step, changing a creature's power at this point does not change how much damage it deals — the assigned values are fixed (CR 510.1c). The power value that mattered was the one at assignment time.

There is one important nuance: if a creature has an ability that modifies how it deals damage (such as Lifelink or Deathtouch, under CR 702.15 and 702.2), those abilities are checked at the time damage is actually dealt, not when it was assigned. So a creature that gains Lifelink after assignment but before damage is dealt will still grant life from that damage.

Concrete example: Your 3/3 creature is assigned to deal 3 damage to a blocker. In response, your opponent casts a spell to reduce your creature's power to 1. Your creature still deals 3 damage — the assignment was already made and is locked in per CR 510.1c.

In summary: power at assignment time determines the damage amount, and that amount cannot be changed afterward even if power changes before damage resolves.