The Senate has been engaged in intense debate over the legislation, a compromise version of the bill reached in the Senate Foreign Relations Committee last week in an effort to avoid a presidential veto.
A year-and-a-half before the 2016 election, presidential politics have also influenced the legislation.
Senators Bob Corker and Ben Cardin, the committee`s Republican chairman and top Democrat, have been arguing against so-called poison pill amendments seeking to toughen the Iran Nuclear Review Act.
They insist those amendments would kill its chances of becoming law by alienating Democrats and provoking a veto. Obama considers tougher restrictions a threat to ongoing nuclear negotiations between Iran and six world powers.
Cardin had a heated exchange with Senator Marco Rubio, a Republican presidential hopeful, on Wednesday over one of the seven amendments Rubio has filed seeking to toughen the bill.
Rubio wants to amend the bill to prevent a nuclear deal from going forward unless Iran`s leaders accept Israel`s right to exist as a Jewish state, a measure certain to provoke a veto threat.
Rubio`s critics have accused him of pushing the measure to enhance his foreign policy credentials as he fights for the White House. Pro-Israel politics are particularly important to evangelical Christian voters, a key part of the Republican base.
But Rubio accused some of his fellow senators of refusing to consider the amendment because they did not want to take a difficult vote.
Cardin, a lawmaker known as a strong supporter of Israel, vehemently disagreed and blasted the amendment as a poison pill. "It would make it almost impossible for the president to negotiate an agreement with Iran," he said.
The Barrasso amendment voted down on Wednesday sought to reinstate a clause that was removed last week as Democrats and Republicans worked out the compromise version of the bill.
Both of the amendments that have come up for a vote so far have been rejected.
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