Can you cast a spell during the declare attackers step before your opponent declares blockers?

After the active player declares attackers, the game moves through the rest of the Declare Attackers Step. Once all triggered abilities from attacking are put on the stack, the active player receives priority first (CR 509.5). Both players may then cast spells or activate abilities before anyone declares blockers.

The Declare Blockers Step doesn't begin until both players pass priority in succession with an empty stack (CR 116.3). So there is a real window — however brief — in which either player can act after attackers are declared but before blockers are declared.

This matters strategically. For example, the defending player might cast a combat trick like Giant Growth targeting one of their creatures, or the attacking player might cast Shock to kill a potential blocker, all before blockers are even chosen.

Keep in mind that casting a spell during this window that takes time to resolve (e.g., a spell with a long stack sequence) can give your opponent additional opportunities to respond as well, since both players continue to receive priority until the stack is empty and both pass (CR 117.4).

Example: You attack with a 2/2. Your opponent has a 3/3. Before your opponent declares blockers, you cast Titanic Growth (+4/+4) on your attacker. Your opponent then declares blockers — but now faces a 6/6 instead of a 2/2, changing their blocking decision entirely.