This was a hard negotiated agreement between many more countries than just the U.S. and Iran. From the get-go, the GOP Congressmen swore it would be a bad deal, no deal should be ratified, and any failings were all the fault of this POTUS. Just like with any and all issues in the past 7 years, the GOP does not want this POTUS to "win" at anything. The GOP would rather see our sons and daughters killed in another ill-thought-out war than agree with the other side of the political aisle.
Few who post on internet discussion boards have read the entire agreement. Most are simply echoing what they have heard from Hannity, Limbaugh, Beck, etc.
What brought a number of the votes to side with the deal was the treasonous actions of a handful of GOP Congressmen. To send a letter to foreign nations saying that whatever our duly elected POTUS and appointed Secretary of State negotiated, it wasn't worth the paper on which written. A GOP Congressman is right now in Israel telling Bibi he will do anything in his power to undermine the agreement. Again... traitorous!!! Rather than seen as siding with these who would undermine the agreement simply to kick sand in the President's face, a number have said they will not support override of the veto.
The sanctions against Iran were already falling apart. Russia was short-circuiting the sanctions and "back-dooring" weapons, etc. to Iran. We have asked Iran to assist in the defeat of ISIS. So far, the enemy of our enemy is still our enemy... but what else is new in the Mid-East????
When the Affordable Care Act was being implemented, we heard all the talk about "death squads" that would kill Grandma. We heard how doctors would be leaving the profession by the dozens. All kinds of gibberish was tossed out in an attempt to undermine that bill. Today, we are hearing issue after issue as to how Iran will now be free to build nuclear weapons and send them our way post haste. Every person setting around that negotiating table has sons and daughters, mothers and fathers, friends and family that would not put in harm's way by purposely constructing an agreement which would lead to the annihilation of the U.S. America will be destroyed far sooner by the constant and consistent criticism of our elected officials... by the desire to rather see the Nation destroyed than the other political party win... than by a nuclear attack from Iran.