Iran has treated this "agreement" like a joke from the start. It's pretty clear that this is just a way for Obama to ease up the pressure in stages because he doesn't have a real plan to stop Iran from getting nukes and isn't about to get the US into a conflict with them.
This is his own party standing up and saying that no, this isn't acceptable.
Because, god forbid, not getting into a conflict with Iran would be a travesty.
I don't want a conflict with Iran, but it's preferable to a total surrender, which is the likely outcome of this deal. Iran gives nothing, and the US gets cover to disengage.
How can we surrender to a country we aren't at war with? Iran having the bomb is far less of a threat to the average American than is a United States Congress or President who decides mass slaughter of American boys and girls is in the interest of the United States.