Can you tap a land for mana after blockers are declared but before combat damage?

Combat is divided into five steps: Beginning of Combat, Declare Attackers, Declare Blockers, Combat Damage, and End of Combat. After each step's active actions resolve, both players receive priority and may take actions — including activating mana abilities — before the next step begins (CR 116.3b, 506.3).

After blockers are declared (Declare Blockers step), there is a round of priority in which players may cast spells or activate abilities. Tapping a land for mana is a mana ability and can be activated any time you have priority (CR 605.3a). This mana then sits in your mana pool, ready to be spent on instants or activated abilities during that same combat window.

This is especially relevant in the Combat Damage step itself: after blockers are declared but before damage is dealt, players can use this window to cast pump spells or activate abilities that require mana — and they can tap lands right then to pay for them (CR 510.1).

Concrete example: You attack with a 2/2. Your opponent blocks with a 2/2. After blockers are declared, you tap a Forest to add {G} to your mana pool, then spend it to activate your creature's +1/+1 ability. Your creature is now 3/3 when combat damage is assigned, surviving and killing the blocker.

In short, mana abilities (including tapping lands) can be activated any time you have priority — and you do receive priority during combat between each step and after each spell or ability resolves (CR 116.3, 605.3a).