Can you cast an instant spell during the declare attackers step before blockers are declared?

After the active player declares attackkers, the game checks state-based actions and then grants priority to the active player (CR 509.4). Because priority is passed during this step, any player who receives priority may cast instant spells or activate abilities before the game moves on to the Declare Blockers Step.

This is a very common and important window. The defending player often wants to cast removal or tricks here — for example, to kill an attacker before blockers are even declared, which can change blocking decisions entirely. The attacking player equally can cast pump spells to boost their attacker's power or toughness at this point.

Priority works the same way in every step and phase that doesn't explicitly forbid it (CR 116.3). Each player must pass priority in succession without anyone taking an action before the game advances to the next step. So both the active player and the defending player get a chance to act here.

Example: Your opponent swings with a 3/3 creature. Before you declare any blockers, you cast Lightning Bolt targeting it. The 3/3 dies, and when the Declare Blockers Step begins, there is no attacker left to block.

Key rules: CR 509.4 (priority after attackers are declared), CR 116.3 (when players receive priority), and CR 507.1 (instant speed actions during combat steps).