This case. The article seemed to refute that
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He's not taking new powers. He's being granted them by Congress.
Yes, Congress hasn't given him all powers, but with this bill, they are giving up the power to offer amendments to any treaty, a power they have long had.
I think that power is one that is detrimental to executive negotiating, but that power has been Congress's. Despite what the article claims, that is what the TPA is doing. (Notice, he doesn't address this point. He only says that Congress still gets to vote on the treaty! Well, yes, but few I've read claimed that they were giving up that power- though I'm sure some were confused.)
It IS a new power, or a diminution of congress's power, however you wish to phrase it.