Barbara Mikulski announced her endorsement of the Iran deal, becoming the 34th senator to voice her support, thus giving the Obama Administration enough votes to block a veto override. In remarks at the University of Delaware, Coons said: “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”.

Leahy says Congressional rejection of the deal would have nearly certainly led to war in the Middle East.

That will allow backers to uphold Obama’s veto, if necessary, of a resolution of disapproval Republicans are trying to pass this month.

To Senator Leahy this is a pretty clear-cut issue: support a deal that has the framework of a plan to monitor Iran’s nuclear program in the future or reject it and risk that Iran will develop a nuclear weapon in the next few years.

Because Republicans, who hold majorities in both the House and Senate, are in almost unanimous opposition to the agreement, the White House has worked to win over skeptical Democrats to ensure Congress can’t block it.

Thirty-two Senate Democrats and two independents who vote with the Democrats now support the agreement. “But the United States must recognize that to make this deal work, we must be more vigilant than ever in fighting Iranian aggression”.

Support for the Iran nuclear deal rose in the U.S. Senate on Thursday as three more Democratic senators, Cory Booker of New Jersey, Mark Warner of Virginia and Heidi Heitkamp of North Dakota, said they would back the agreement.

Kerry also said that if the USA rejects the deal, it would confirm the fears of Iran’s leaders “that you can’t deal with the West, that you can’t trust the West”. That would mean members could block the resolution of disapproval from coming to a vote in the first place and spare Obama the need to break out the veto pen. The president spoke personally to about 100 lawmakers, either individually or in small groups, and aides said he called 30 lawmakers during his August vacation on Martha’s Vineyard.

The milestone brings an end to a period of uncertainty for all the players involved in the deal, where it seemed possible that an agreement painstakingly negotiated in Vienna could be unraveled by a vote in Washington. But Iran has stonewalled worldwide inspectors in the past and the US does not have “absolute knowledge”, according to a former Obama administration official, Robert Einhorn.