The potential upside of legislating further sanctions is the hope that increased pressure might elicit more concessions or push Iran to conclude a more favorable deal. But this is unlikely. The potential downside is more likely and more dangerous: Iran’s decision makers could conclude that the United States government was not negotiating in good faith — a view that Iranian hard-liners already espouse. This could prompt Iran to walk away from the negotiations or counter with a new set of unrealistic demands while redoubling its efforts to produce nuclear weapons.
Instead of slowing Iran’s nuclear program, such legislation could actually accelerate its quest for atomic weapons, leaving a stark choice: Either accept the possibility of a nuclear-armed Iran, or use military force to stop it.
Worse still, it could alienate our international partners. The sanctions have been effective largely because of the active participation of many countries, including China and Russia. When the United States alone doesn’t buy Iranian oil, it has little effect on Iran’s economy, but when the European Union stops, and other major oil customers of Iran such as China, Japan, South Korea, India and Turkey significantly reduce their purchases (which they have), Iran is in trouble (which it is).