Does static ability from Anthems apply in the declare blockers step or only at the start of combat?

Static abilities like those on "anthems" (e.g., Glorious Anthem — "creatures you control get +1/+1") create continuous effects that are always active as long as the permanent is on the battlefield. They do not "turn on" at a specific step. This is governed by CR 604.1, which states that static abilities are applied continuously and don't use the stack.

This means the power and toughness boost from an anthem is present during the declare blockers step (CR 509), just as it is during the declare attackers step, the combat damage step, and even on your opponent's turn. There is no special timing window where the bonus disappears or hasn't yet applied.

Importantly, if an anthem enters the battlefield during the declare blockers step (e.g., via a flash creature or instant-speed effect), the bonus applies immediately to all qualifying creatures — including those already declared as blockers. The new continuous effect is evaluated right away per CR 613 (the layer system).

Concrete example: You attack with a 2/2 creature. Your opponent blocks with their own 2/2. You cast Glorious Anthem before damage is dealt. Your attacker is now a 3/3. When combat damage is assigned in the combat damage step, your 3/3 deals 3 damage to the opposing 2/2, destroying it, and your creature survives with 2 damage (since it now has 3 toughness).

So there is no "only at the start of combat" restriction — anthems are continuous effects active at every moment the source permanent is on the battlefield (CR 604.1, CR 613).