President Obama's Iran Nuclear Deal Officially Unstoppable in Congress

— -- President Obama's landmark Iran nuclear agreement will survive congressional review.

Sen. Barbara Mikulski, D-Md., has announced her support for the Iran nuclear deal, becoming the 34th Democratic senator to back the president, and giving Obama the numbers in the upper chamber to sustain his promised veto of the resolution of disapproval of the deal.

“No deal is perfect, especially one negotiated with the Iranian regime. I have concluded that this Joint Comprehensive Plan of Action is the best option available to block Iran from having a nuclear bomb. For these reasons, I will vote in favor of this deal. However, Congress must also reaffirm our commitment to the safety and security of Israel,” Mikulski said in a statement.

Congressional opponents of the deal would have needed a two-thirds majority in both the House of Representatives and the Senate in order to override Obama’s veto of the Resolution of Disapproval, which would have prevented the administration from lifting some Congressional sanctions on Iran.

But Mikulski’s announcement means opponents will not be able to reach the 67-vote threshold needed to override the veto.

Even as her support sealed the survival of the deal in Congress, the Obama administration didn't start running any victory laps. Shortly after Mikulski’s announcement, Secretary of State John Kerry delivered a passionate speech defending the deal, showing the administration was still continuing to pressure undecided lawmakers.

“Without this agreement, the Iranians will have several potential pathways to a bomb. With it, they won’t have any,” Kerry said at the National Constitution Center in Philadelphia.

House Republicans indicated they didn't think the fight was over, either. A spokesman for House Speaker John Boehner of Ohio said in a statement that the deal remained "far from implemented," although he didn't hint as to what Republican strategy would be to block the deal’s going forward.

House Republicans have enough votes initially to pass the resolution of disapproval through the chamber, but it remains unclear whether they have the Democratic support to overturn the president's veto. Just 14 House Democrats have said they will vote against the agreement, while nearly 100 plan to support it.

It is also unclear whether the House or the Senate will vote first on the resolution of disapproval when Congress returns from its August recess.

Mikulski, the Maryland senator who has served longer in Congress than any other woman in history, is retiring after this term, suggesting it was less politically toxic for her to be the so-called “decisive” vote in favor of the deal than some of the other Senate Democrats who have not yet announced their position on the agreement.

There are 10 remaining Senate Democrats who have not yet made their opinion public, including some who are facing difficult re-election races and others who have large anti-deal constituencies.

But if White House officials can get seven of those 10 holdouts to support the deal, they will not only be able to prevent the resolution of disapproval from going into effect but also be able to sink it upon the first Senate vote, which is expected to happen shortly after Congress returns from the summer recess Sept. 8.

While Congress can no longer scuttle the deal with a vote, lawmakers in both chambers are working on follow-up sanctions legislation that could target Iran for non-nuclear activities, including its sponsorship of terrorist groups, the Syrian regime and human rights abuses.

Members on both sides of the agreement have suggested the United States also provide additional military and technological support to Israel in light of the deal, which Israel strongly opposes.

In letters to lawmakers Wednesday, Kerry outlined security commitments to Israel and the Gulf States, including plans to increase missile defense system funding to Israel.

ABC News’ Alex Mallin contributed to this report.

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