I think Ernie summed it up, though suggesting Obama wants Iran to have nuclear weapons, puts him in the nutter stands. But, the rational people opposing this deal in particular think we can get a better deal, in which Tehran will give up a nuclear program entirely, and that Iran can be persuaded to do this with more sanctions. Jeffrey Goldberg terms this deal "the least worse option" doubts this premise, and so do I, but for reasons a bit different from his.
"War would not end the Iranian nuclear challenge. Israeli strikes, and certainly U.S. strikes, could destroy much of the physical infrastructure of the Iranian nuclear program, but cruise missiles cannot destroy knowledge, and I doubt they could destroy the will of the regime. Iran would rebuild those facilities, and would do so free from the burden of international sanctions, which would most likely crumble after such a preventative attack. Sanctions, too, were not bringing Iran to total capitulation. I do think that sanctions concentrated the attention of the regime, and that additional sanctions might have helped improve America's negotiating position. But the Iranian regime was not going to capitulate in the face of a collapsing economy. It would have adapted."
The Iran Nuclear Deal Obama Chooses the Least-Worst Option The Atlantic
I recall the sanctions on Iran, and those sanctions really did shut down Saddam's nuclear program, and the threat of war allowed inspectors to have free access, which the present framework for an agreement requires. Yet, even when el-Baradi found no evidence of a nuclear program, the GD neocons invaded the place anyway, and in doing so we allied the Iraqi Sunnis with Al Queda. So, I don't believe for a second that ALL the pols saying "we need tougher sanctions" will ever support any agreement, and just want to opt for war, and the sooner the better. Iraq would be a walk in the park compared to Iran, and we really didn't win in Iraq.
France and Germany both say they haven't seen enough from Iran to support withdrawing any sanctions. Sadly, I'm sure they're saner than Cotton of Boom Boom McCain, and I suspect they'll require more from Iran that would Obama.