Anthony begins his turn by drawing a card. He then immediately turns his creatures sideways and declares "I attack with Runeclaw Bear and Alpine Grizzly."

Anthony's opponent, Nathan, has 6 life. Nathan has Crippling Chill and Kill Shot in hand, and the mana available to cast either but not both. He has not yet acknowledged Anthony's shortcut to move to combat, nor has he denied it.

Which of the following is true?

1. Nathan must immediately deny Anthony's shortcut.
2. Nathan may deny the shortcut after pausing to think about what he wants to do.

I am specifically interested in two cases: Regular REL and Competitive REL. If the answer depends on which one is in effect, please cover both. I am not interested in casual play.

Anthony begins his turn by drawing a card. He then immediately turns his creatures sideways and declares "I attack with Runeclaw Bear and Alpine Grizzly."

Anthony's opponent, Nathan, has 6 life. Nathan has Crippling Chill and Kill Shot in hand, and the mana available to cast either but not both. He has not yet acknowledged Anthony's shortcut to move to combat, nor has he denied it.

Which of the following is true?

1. Nathan must immediately deny Anthony's shortcut.
2. Nathan may deny the shortcut after pausing to think about what he wants to do.

I am specifically interested in two cases: Regular REL and Competitive REL. If the answer depends on which one is in effect, please cover both. I am not interested in casual play, but you can certainly cover that if you want to.