The US legislature is a bicameral parliament and any bill must be approved by both houses of the parliament before being signed into law by the president. Apart from laws related to US government's revenues, which are simply discussed and approved by the House of Representatives, and vetting qualifications of presidential hopefuls as well as ratification of international treaties, which are among exclusive powers of the Senate; in other cases, all bills must be passed by the majority of both houses of parliament before they could turn into law.
One of the features of the parliamentary system in the United States of America is lack of powerful partisan discipline in the course of decision-making process. Many bills are voted negative or positive by members of both Democrat and Republican parties and the partisan solidarity that we see in Europe does not exist in the United States.
However, when it comes to opposing Iran's nuclear deal with the P5+1 group of countries – which is known as the Joint Comprehensive Plan of Action (JCPOA) – the Republican members of Congress have shown great partisan solidarity! At the same time , a great number of the Democrat members of Congress are still in doubt about whether to support of oppose JCPOA.
The election win of the Republican lawmakers in the mid-term vote for the Congress in 2015, helped the opponents of President Barack Obama's administration to sway the majority in both houses of parliament. At present, there are 435 lawmakers at the House of Representatives of whom 246 lawmakers are Republican and 188 are either Democrat or independent members. Therefore, a relative majority at the House of Representatives (which means half of the parliamentary seats plus one) is against the Vienna agreement.
However, to prevent the US president from using his right of veto against their decision, that decision should be backed by at least two-thirds of the lawmakers, which amounts to 290 representatives of the parliament.
According to the latest figures released by the CNN news network, 11 out of 188 Democrat members of the House are against JCPOA. If this figure is added to the number of Republican representatives (almost all of whom are absolutely against JCPOA), it would follow that 257 members of the House are against the nuclear deal with Iran. Therefore, to prevent Obama from exercising his veto right, 33 more Democrat representatives must join the opponents of the deal. However, it seems that such an addition, if not impossible, would be very unlikely.
At present, out of 100 senators at the US Senate, 54 are Republican, 44 are Democrat and two senators are independent members of the Senate. All Republican senators are almost absolutely or relatively opposed to JCPOA. Republicans, however, need 13 votes from Democrats at the Senate (to a total of 67 votes) in order to pass a decision which could not be vetoed by the president; however, they have only garnered two votes from the Democrat side of the Senate.
Out of prominent Democrat senators, Chuck Schumer and Bob Menendez have openly announced that they would vote against the nuclear agreement. Therefore, Obama is now sure about 56 negative votes from senators to JCPOA. In order to veto their negative vote, Obama needs 34 positive votes, but according to CNN's report, the number of positive votes currently stands at only 30.
Therefore, in order to exercise his veto right against Senate's decision, Obama would need to get the votes of 4 out of 12 Democrat senators who are still in doubt about JCPOA and the dominant view is that Obama will have no difficulty in getting their votes. At the moment, Republican senators have lost hope in passing a bill against JCPOA, which would not be vetoed by the president, and most of them are even concerned about filibuster by Democrats at the Senate.
Now, assume that heavy investment and serious activities by pro-Israel lobbies that are against JCPOA, some Arab countries, Republican lawmakers and US neocons will finally succeed to cause JCPOA be rejected by the US Congress in such a way that even the president would not be able to veto their decision. In that case, what would happen to JCPOA and Iran's nuclear agreement with the West? It seems that three scenarios are predictable in this regard.
First scenario: This scenario forecasts a pessimistic situation in which conditions will revert to where they stood before the Geneva agreement of November 24, 2013, and even to conditions that existed before the election of Iranian President Hassan Rouhani's administration. In that case, sanctions against Iran would be reinstalled and new sanctions would be put on the agenda. Iran, on the other hand, due to breach of the agreement by Americans, will resume its peaceful nuclear activities and may even reduce cooperation with the International Atomic Energy Agency to a minimum. If Rouhani's administration does not collapse, chances for its reelection in the next poll, which is scheduled for two years from now, would be infinitesimal. On the other hand, in the event of the election of a Republican president in the United States, the final outcome of this scenario would be what Obama has already predicted: war.
Second scenario: In view of the United Nations Security Council Resolution 2231, which backs JCPOA, and given the positive political will in Europe and across the world – save for the United States and Israel – toward Iran's nuclear deal, apart from sanctions imposed by the US Congress, all other sanctions – even those imposed according to US president's executive orders – would be gradually lifted and a serious gap would appear between the United States and Europe on how to deal with Tehran. A similar gap existed between the two sides under Iran's former president, Mohammad Khatami. Even if the trigger mechanism – as stipulated in articles 11 and 12 of the Security Council Resolution 2231 – is used, the regime of international sanctions would fall apart. Most probably, this option would be the strategy of choice for Rouhani's administration, civil and political society, as well as the reformist and rational principlist politicians in Iran.
Third scenario: This scenario predicts conditions will become similar to circumstances that existed from November 24, 2013 to July 14, 2015. This means that nuclear talks would be resumed and the United States would give small concessions to Iran such as extension of the primary Geneva agreement (reached in 2013) in the hope of taking major concessions from Tehran. At any rate, a warmongering policy is not accepted by the majority of the American people right now. On the other hand, apart from war or suffering more sanctions, Tehran would have no other choice but to accept Washington's offer and the situation would be similar to conditions that existed between Iran and the West from November 24, 2013 to July 14, 2015. This scenario may be acceptable to the United States and Israel, but it will have no place in Iran's power structure and will give rise to a new form of radicalism in Iran's domestic and foreign policies as a result of which JCPOA would totally collapse. In the meantime, countries like China and Russia and even the European Union would not be willing to restart such tiring negotiations from scratch.
Political developments in Iran have led to strong support of the Iranian public for Rouhani's policies, while developments in international politics have caused world powers to be now averse to the aggressive policies of Israel and the US neocons. This is why British Foreign Secretary Philip Hammond said when answering a question posed by an opposition member of the parliament that "the question you have to ask yourself is what kind of a deal would have been welcomed in Tel Aviv? The answer, of course, is that Israel doesn't want any deal with Iran. Israel wants a permanent state of standoff and I don't believe that's in the interest of the region." Therefore, even if Obama would not be able to veto possible decision of the Congress against JCPOA, the most probable scenario to unravel would be the second scenario.