ATs Button - isolating LP

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I cap the flop and see his reaction on the turn.

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A loose/passive calls the raise preflop, granted it's a standard "in for one, call for two" type of a thing. Then he bets into you on the flop, no doubt knowing you'll most likely raise, followed by the three bet???

I think this is pretty serious play from an LP.

What range are you putting him on and what "reaction" are you looking for?