Don't you think that an executive agreement rather than a treaty should be
preferable to those opposed to the deal (assuming, of course, that a treaty could be ratified by the Senate)? The reason I say that is because an executive agreement is far easier to undo later than it would be to rescind or alter a treaty. So, whether for political reasons, or because we find out later that Iran is not living up to its commitments in the EA, the next President would be able to easily nullify the EA. Rescinding or terminating a treaty, on the other hand, can get really, really messy, even if the President has the full backing of the Senate.
The Supreme Court held (OK, technically not a holding, at least not in the stare decisis sense, right?) in Goldwater v. Carter that the question of whether a President can unilaterally nullify a treaty is a political one, and therefore not justiciable. However, nullifying/terminating a treaty, whether by presidential fiat or with Senate approval, raises issues under international law, the Vienna Convention, etc.
And, for the record, I firmly believe that Iran will have a nuclear weapon within 10 years--regardless of whether the EA is entered into, and regardless of who the next President is. I'll but you a beer in 2025 if I am wrong.