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

When the declare blockers step begins, the game first checks for any triggered abilities that triggered at the start of the step, then both players receive priority in turn order (CR 509.1). During this initial priority window, any player may cast instants or activate abilities before the defending player is required to declare blockers.

Blockers are not declared until the active player and all other players have passed priority in succession without taking any action (CR 509.1). Only at that point does the defending player declare which creatures will block and which attackers they block (CR 509.1a). Until then, the game is in a normal priority window.

This means the attacking player gets priority first and can cast an instant — such as a pump spell or removal — before the defending player even chooses blockers. The defending player also gets a chance to respond before blockers are locked in.

Example: You attack with a 2/2. Before your opponent declares blockers, you cast Giant Growth targeting your attacker, making it a 5/5. Your opponent must now decide whether to block the 5/5, not the original 2/2. After your spell resolves, priority passes back and blockers are then declared.

Relevant rules: CR 509.1 governs the declare blockers step procedure, and CR 116.3 covers when players receive priority during a step or phase.