Critics of the senators' plan argue that new sanctions right now would (1) violate the United States' promise not to impose new sanctions as part of the current interim deal, making it much likelier that the deal will fall apart; (2) be seen as punishing Iran for cooperating with us, which is the opposite of the message we want to send; (3) weaken pro-diplomacy forces within Iran and strengthen hard-liners who oppose cooperation; (4) give Iran an excuse to not only walk away from the table but also to then frame the U.S. as the intransigent party, potentially weakening the global sanctions regime against Iran.
In touching on some of these points, Stewart deploys his favored tactic of using old video to show how a politician's previous statements seem to contradict his or her current position. Sen. Lindsey Graham (R-S.C.), who supports the new sanctions, had previously argued that sanctions would bring Iran to the negotiating table (that's also been the Obama administration's position). So now that they're at the table, haven't sanctions accomplished their purpose?
To be clear, Stewart is coming at this from the perspective of a liberal who supports the Iran deal, which is a common but far from universal view among foreign policy professionals. But, and this is the thing, even a number of people who are skeptical of the deal itself still see the sanctions push as self-defeating. Bloomberg's Jeffrey Goldberg, in a column titled "An Iran Hawk's Case Against New Iran Sanctions," warned that the sanctions would "move the U.S. closer to war with Iran and, crucially, make Iran appear -- even to many of the U.S.'s allies -- to be the victim of American intransigence, even aggression." All without actually denuclearizing Iran, which is the senators' stated goal.
I do think Stewart is misguided on one very important point. He asks, reasonably: Why are 59 senators pushing for a sanctions bill that most observers seem to think will actually set back the bill's goal? He argues that this is explained by the nefarious behind-the-scenes lobbying of pro-Israel groups such as the American Israel Public Affairs Committee. AIPAC's supposedly vast power is certainly conventional wisdom here in D.C. In practice, though, the group is highly successful at lobbying Congress to pass popular legislation that carries a low political risk and is much less successful at getting unpopular legislation passed. Stewart is right to argue that it would be absurd to give Israel a vote in Congress; fortunately, that's not what's happening here.
As I've written previously, it seems much more likely that senators are supporting Iran sanctions because doing so is squarely in their political self-interest. Iran is very unpopular in the United States. Republicans are already signaling that they may use the Obama administration's Iran outreach as a weapon in coming elections. Any lawmaker who votes against new sanctions, even if it is for very sound foreign policy reasons, is taking a big political risk. That's doubly true if the Iran deal fails. And if the Iran deal succeeds, all the credit will go to the Obama administration, so there's little political incentive for individual members of Congress to support diplomacy.