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Can you cast an instant that gives creature(s) +X/+X until end of turn /after/ you've declared attackers /and/ your opponent has declared blockers? E.g. Tandem Tactics or Swell of Growth

marked as duplicate by Benjamin Cosman, ConMan, Michael Snook, Nij, Ivar de Bruin Mar 1 '17 at 7:13

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Yes. Players get "priority" - the chance to use spells and abilities - in between most events of the turn, including between when blockers are declared at the beginning of the Declare Blockers Step and when they deal damage at the beginning of the Combat Damage Step. (You can also boost your creatures' power even later in the turn such as during your Second Main Phase, but I assume you're trying to have them deal more combat damage so you'd better do it before the Combat Damage Step begins.)

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