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

Once the active player declares all attackers and the game checks for any 'whenever a creature attacks' triggered abilities, both players receive priority in the Declare Attackers Step. Any player who has priority may cast instants, activate abilities, or do anything else they could normally do at instant speed. (CR 509.4, 116.1)

Sorcery-speed spells — including sorceries themselves and creatures (unless you have flash) — cannot be cast here, because it is not your main phase and the stack is not empty. Only instant-speed actions are permitted during combat steps. (CR 307.1, 302.1)

This matters a great deal in practice: a defending player can cast a combat trick like Giant Growth or a removal spell like Doom Blade on an attacking creature before the defending player even declares blockers, potentially changing combat math or killing a creature before blocking is even considered.

Example: Your opponent attacks with a 2/2. Before you declare blockers, you cast Doom Blade targeting that 2/2. It dies during the Declare Attackers Step, so no blocking decision needs to be made at all.

Priority continues to pass back and forth until both players pass in succession with an empty stack, at which point the game moves to the Declare Blockers Step. (CR 117.4, 509.4)