Bunker, as usual, has got it all backwards. The treaty with Iran wasn't intended to allow them to develop a nuclear weapon. The Treaty was intended to prevent them from developing a nuclear weapon, because inspections. It's not having a treaty with Iran, which eliminates inspections, that will allow Iran to develop a nuclear weapon.
Fact is, right wing hacks, Putin fanboys, and other Republicans need the specter of nuclear Iran as a cudgel to clobber Democrats with during election cycles. Without that—that is, with a treaty in place—they have nothing of substance on Iran and can't effectively attack Democrats on this issue.