Can you use a mana ability to add mana after declaring attackers but before blockers are declared?

After attackers are declared, the active player receives priority in the Declare Attackers Step before the game moves to the Declare Blockers Step. During this priority window, any player may activate mana abilities to add mana to their mana pool (CR 509.4).

Mana abilities are special: they don't use the stack and can be activated any time you could activate an instant-speed ability (CR 605.3a). Since the active player has priority after attackers are declared, they — and eventually the defending player — can activate mana abilities freely before the game advances to the Declare Blockers Step.

This is commonly relevant when a player wants to hold up mana for a combat trick or instant-speed spell during the combat phase. You can tap a land or activate any other mana ability right in that window to prepare for upcoming responses or spells.

Example: You attack with Grizzly Bears. After attackers are declared, you tap a Forest to add G to your mana pool before your opponent declares blockers, so you have mana ready to cast Giant Growth if needed during the Declare Blockers Step or later in combat.

Note: The mana will empty from your pool at the end of each step if unused (CR 500.4), so timing matters — but yes, activating a mana ability at that precise moment is fully legal.