Yes, it's the "beginning of combat step". Declaring attackers is a turn-based action that happens at the very beginning of the "declare attackers" step and no one can interrupt it once it starts. If you want to do anything to affect how attackers are declared you have to make your play before the "beginning of combat" step ends.
September 11, 2017 2:07 p.m.
To expand on what Rhadamanthus said, you have to make your play when you have priority during your opponent's beginning of combat step.