Can you use a land's mana ability after attackers are declared but before blockers?

After attackers are declared in the Declare Attackers Step, both players receive priority in turn order before the game moves to the Declare Blockers Step. During this priority window, any player may activate mana abilities — including tapping a land for mana — because mana abilities don't use the stack and can be activated whenever you could activate an instant or a mana ability (CR 605.3a).

Mana abilities are special: they resolve immediately without going on the stack (CR 605.3b). This means you can tap a land for mana at essentially any point you have priority, which includes right after attackers are declared and before the defending player declares blockers.

This is commonly relevant when you want to hold up mana for a combat trick (like Giant Growth) or a flash creature you might cast during combat. You can tap your lands for mana in the Declare Attackers Step window and still spend that mana during the Declare Blockers Step or the Combat Damage Step, as long as the mana remains in your mana pool.

Example: You attack with a 2/2. After attackers are declared, you tap a Forest for one green mana (mana ability, no stack). Your opponent then declares a 3/3 as a blocker. You now spend that green mana to cast Giant Growth targeting your attacker, pumping it to 5/5 and winning the fight.