The Iranian nuclear deal is poised to clear remaining political hurdles in Washington after key Senate Democrats indicated there was now enough support in Congress for Barack Obama to withstand any Republican-led effort to block it.



Senator Bob Casey of Pennsylvania, regarded as a critical swing vote, became the 32nd Democrat to declare his support on Tuesday for the deal, which will ease sanctions on Iran in exchange for steps aimed at preventing it from developing a nuclear weapon.



“I believe that this is better for our security and better for Israel’s security, without a doubt, short term and long term,” Casey said in an interview with the Philadelphia Inquirer.



Casey’s decision to endorse the agreement came against the backdrop of roughly $1.3m in advertising on Philadelphia’s television networks this summer alone, from groups on both sides of the debate – but with time booked by critics dwarfing that of proponents of the deal.



Coming out for the deal, reached in July by Iran and six world powers, was “one of the most difficult decisions of my public career”, Casey said.



Senator Chris Coons of Delaware also declared his support for the deal in a speech on Tuesday and in an interview with the Washington Post.

“I will support this agreement and vote against any measures to disapprove of it in Congress,” concluded Coons after providing a lengthy list of his remaining concerns about the deal.



“It puts us on a known path of limiting Iran’s nuclear program for the next 15 years with the full support of the international community. The alternative to me is a scenario of uncertainty and likely isolation.”



“I will support this agreement despite its flaws because it is the better strategy for the United States to lead a coalesced global community in containing the spread of nuclear weapons,” he added.



Earlier, Ben Cardin, the top Democrat on the Senate foreign relations committee, predicted the White House would now reach the magic number of 34 senators – needed to prevent Congress overriding a presidential veto – by the weekend.



“The numbers in Congress are looking pretty close to favouring the president,” he told students at Johns Hopkins university in Baltimore on Tuesday morning.



“In the US Senate, the president will have at least 32 committed Democrats on his side by the end today and it looks like we will clearly get to the 34 number by the end of the week,” added Cardin, who has not yet declared his own position. “So it looks pretty clear that the president is going to have the support to sustain a veto.”



Just two Senate Democrats – Chuck Schumer of New York and Bob Menendez of New Jersey – have publicly opposed the deal thus far. Both senators have strong ties to the American Israel Public Affairs Committee (Aipac), which is leading the charge against the agreement in Washington, and were widely expected to break with Obama given their prior criticisms of the negotiations.



Cardin said the level of lobbying had been “aggressive” and criticised some of the vitriol directed both at politicians in favour and against the deal. “It’s been intense,” he told the Guardian. “Most [of it] is healthy but there have been some people on both sides who have gone further than is useful.”



Under the terms of an oversight process agreed by Cardin and the foreign relations chair Bob Corker, Congress has until 17 September to review the deal struck between Iran and a group of international powers including the US and Britain.



Even if both Senate and House pass a motion of disapproval, however, it can still be vetoed by the president, who retains the right to conduct international negotiations. Only a vote of two-thirds of the members of both the US Senate and House of Representatives can override a presidential veto, which is why the support of 34 senators marks the effective finishing line.



“If we can’t get two thirds of the vote, then under the rules of the game – the rules called the constitution of the United States – then the deal goes forward,” said Cardin. “And at that point, it’s critically important for Congress to rally behind the president so that he can carry it out the best that he can.”



The avalanche of Democrats coming out to support Obama’s decision on the deal in recent days raises the prospect that Republicans may fail to pass a vote of disapproval at all.



Sixty senators would be needed bring debate to a close and pass the motion of disapproval in the first place, so if 41 Democrats come out in favour, the president will not have to use the veto at all.

