Former director for Iran at the U.S. National Security Council and a member of the U.S. nuclear negotiating team with Iran Richard Nephew (he was in the negotiating team from August 2013 to December 2014) wrote an article in CNN commenting on why he believes the US should restrain from new sanctions against Iran. In the article Richard Nephew explains his position by presenting the following counterarguments to the main arguments brought by supporters of the new legislation that presupposes additional sanctions against Iran.
Argument N1: Introducing additional sanction the US Congress will have its say in the negotiations between Iran and the six.
Counterargument: According to Nephew the US Congress has always had and is to have its say in negotiations. As a former negotiator Nephew assures the US negotiators spend as much time discussing the issues on the table with the congressmen as with the Iranians. Accordingly all the issues raised by the US Congress are being presented in the negotiation table; as Richard Nephew argues not only by the US representatives, but also by the representatives of other countries in the format 5+1.
Argument N2: Introducing additional sanctions the US Congress will prevent "bad deal".
Counterargument: Nephew posits that the US congress has other ways to prevent "bad deal", which would be less provocative and would not risk collapsing negotiations completely. For example, the US congress can refuse to finance the implementation of such deal after negotiating parties reach consensus on it.
Argument N3: The US need to reassert its strength by pushing new sanctions against Iran.
Counterargument: Richard Nephew argues that so far the sanctions introduced by the US against Iran has demonstrated the strength and power of the US and now Iran has to go to a great number of concessions to recover its economy from negative impacts of those sanctions.
Conclusively, Richard Nephew states that the US congressmen by their efforts to force the Iranian side to publicly cross "read lines" will have the opposite effect: it will become even more difficult for the Iranians to go to concession and sell those concessions to the Iranian hard liners and public. Thus, according to Nephew, new sanctions will strengthen those hardliners in the country giving them additional justification for their tough positioning in ongoing negotiations.