According to the CIA Factbook, Iran is a theocratic republic. The supreme leader, Ayatollah Ali Khameni, overseas the military and judiciary branches in Iran. Therefore it is dishonest for Mr. Obama and Secretary Kerry to pretend that we've made great strides in reigning in the Iranian nuclear program just because Iranian President Rouhani likes the draft agreement.
Mr. Obama and his fellow Democrats like to criticize George W. Bush for portraying Iraq as a threat that it really wasn't to the US. However, Mr. Obama and Kerry (along with Presidential hopeful Hillary) have done the same thing with Iran. Even if Iran were able to produce enough weapons grade plutonium to build a warhead, that's only half of the battle. They still have to produce a weapon system capable of striking the US (and in enough numbers to defeat our defenses). That it is an extremely tall order and it is the same reason why the US has typically not worried when North Korea starts to rattle their nuclear saber.
The real nuclear threat to the US is the one Mr. Obama keeps poking, Russia. Mr. Obama is hellbent on reducing the military overall but especial the nuclear weapons in the US inventory. Russia is developing a new type of nuclear cruise missile, the KH-101. The problem for the US is the sheer number of these weapons (launching from manned bombers as well as submarines) could easily overwhelm US defenses (unlike the hypothetical threat of a nuclear Iran). Mr. Obama has cut and demoralized the US military to the point that this is a very real scenario. Couple that with his penchant for leveling economic sanctions against Russia and we are on very thin ice.
As I've written previously, Iran has no reason to trust the US. The Iranian citizens see the US as trying to make their country irrelevant through economic sanctions. Having a nuclear weapons program is the only thing the west respects in their eyes. (While we are on this, consider that the US is always leveling economic sanctions but when was the last time you've hear of someone trying to level a sanction against the US? So much for fair play!)