WASHINGTON -- President Barack Obama secured a landmark foreign policy victory Wednesday as Democrats amassed enough votes to ensure the Iran nuclear deal survives in Congress, despite ferocious opposition from Republicans and Israel's government.

The backing from Democratic Sen. Barbara Mikulski gives supporters of the agreement the crucial 34th vote in the 100-member chamber they need to uphold an Obama veto of a congressional resolution of disapproval if Republicans pass such a measure later this month.

And it spells failure for opponents of the international agreement who sought to foil it by turning Congress against it. Leading that effort were Israel and its allies in the U.S., who failed to get traction after spending millions of dollars trying.

The agreement signed by Iran, the U.S. and five other world powers limits Iran's nuclear program in exchange for hundreds of billions of dollars in relief from international sanctions. Republicans and Israeli officials contend that concessions made to Iran could enable the country to wreak havoc throughout the Middle East.

Prime Minister Benjamin Netanyahu, who had personally lobbied U.S. lawmakers to block the nuclear pact, will continue fighting the agreement, an Israeli official said.

Marshall Wittmann, spokesman for the pro-Israel lobby AIPAC, said his group also would continue rallying opposition to the nuclear agreement.

Senate Majority Leader Mitch McConnell, a Republican, grudgingly acknowledged that his side would not be able to block the deal after Obama, in his words, secured "the tepid, restricted and partisan support of one-third of one house of Congress." McConnell spared the accord no criticism, saying it leaves Iran "with a threshold nuclear capability."

In a letter delivered to Congress Wednesday, Secretary of State John Kerry called Israel's security "sacrosanct," recounting the billions of dollars the U.S. has provided the Jewish state for missile defence and other security assistance. U.S. and Israeli officials, he said, are working on a deal to "cement for the next decade our unprecedented levels of military assistance."

The letter was sent as Kerry defended the Iran deal in Philadelphia. His speech was carried live on television in Iran, an unusual occurrence.

"Rejecting this agreement would not be sending a signal of resolve to Iran, it would be broadcasting a message so puzzling that most people across the globe would find it impossible to comprehend," Kerry told lawmakers and civil leaders at the National Constitution Center.

"It's hard to conceive of a quicker or more self-destructive blow to our nation's credibility and leadership -- not only with respect to this one issue, but across the board, economically, politically, militarily, even morally. We would pay an immeasurable price for this unilateral reversal," Kerry argued.

He said the Obama administration would ensure that America's Arab allies, like Israel, would have the "political and military support they need" to protect themselves from the threat posed by Iran.

White House press secretary Josh Earnest called the growing support a validation of Obama's effort to "make sure that every member of the Senate understands exactly what's included in the agreement." The deal sets Iran back so that it is at least a year away from being able to produce enough nuclear material for a weapon, before the restrictions ease after a decade.

With opposition to the agreement failing to take hold on the Democratic side, supporters may be able to muster the 41 votes needed to block the disapproval resolution from passing in the first place, sparing Obama from having to use his veto pen. That would require seven of the 10 remaining undeclared senators to decide in favour of the deal.

Even if Congress passes the disapproval resolution, it can't stop the deal reached by Iran, the United States, Britain, France, Germany, Russia and China. It might help congressionally mandated sanctions remain in place. But the U.N. Security Council endorsed the nuclear deal unanimously in July and outlined how it would lift international sanctions on Iran.

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Lee reported from Philadelphia.