Secretary of State John F. Kerry defended the international nuclear deal with Iran as a step toward a safer world Wednesday, even as the Obama administration secured enough backing in the U.S. Senate to carry out the agreement.

"History may judge it a turning point, a moment when the builders of stability seize the initiative from the destroyers of hope," Kerry told an invited crowd of about 200 at the National Constitution Center.

Before Kerry spoke, Sen. Barbara Mikulski (D.,Md.) announced her support for the accord. She was the 34th senator to endorse it, meaning that President Obama would have enough votes to sustain a veto if Congress rejects the plan.

Republicans remain united in opposition to the deal and are promising to move ahead with a resolution of disapproval in the House and Senate as early as next week. Administration officials will lobby the 10 uncommitted Senate Democrats in hopes of getting 41 commitments needed to filibuster the measure.

The deal with Iran has sparked the most divisive foreign-policy debate in the U.S. since the decision to invade Iraq in 2003. Polling is mixed.

The government of Israel opposes the agreement, fearing that Iran could still build a bomb and wipe out the country. U.S. Arab allies in the Gulf also are nervous about a resurgent Iran, with billions in new economic activity due to the lifting of international sanctions.

Reflecting the stakes, several protestors gathered outside the Constitution Center to denounce the deal.

"I fear for the safety of myself, my family, my neighbors," said Steve Feldman, executive director of the Philadelphia-area chapter of the Zionist Organization of America. "I can't imagine during World War II, President Roosevelt giving $56 billion to Nazi Germany or Japan. . . . That's what this president is doing."

Others among the half dozen protestors who gathered on the sidewalk in the heat and humidity agreed.

"You're making a deal with the devil," Gene Shusman, of Jenkintown, said. "Israel is on the chopping block."

Kerry, a former Massachusetts senator and the Democratic presidential nominee in 2004, made a series of practical arguments in favor of the deal, noting that Iran was close to building nuclear weapons even under economic sanctions in 2013.

The regime of restrictions and inspections negotiated with Iran by the U.S. and five other world powers would prevent the Islamic Republic from producing weapons-grade plutonium, Kerry said. He said it would reduce the country's centrifuges from 10,000 to 3,500 and shrink its stockpile of enriched uranium from 12,000 kilograms to 300 kilograms - not enough to build weapons.

The secretary of state said the U.S. has not given up any option, including military action, in the event Iran were to breach the agreement.

"President Obama has been crystal clear we will do whatever is necessary to keep Iran from getting nuclear weapons," Kerry said. "But the difference is we wouldn't have the world behind us as we have today. The deal "sets us on the road to greater stability and security but doesn't require us to give up any option at all," he said. "To vote down this agreement is to solve nothing."

Kerry said it would be impossible for Iran to hide attempts to develop a nuclear weapon, even though the agreement gives a 24-day warning before on-site inspections.

"In truth, there is no way in 24 days, or 24 months - or 24 years, for that matter - to destroy all the evidence of illegal activity that has been taking place regarding fissile material," he said. "You can't eliminate the evidence by shoving it under a mattress or flushing it down a toilet or carting it off in the middle of the night."

He said the U.S. commitment to Israel's security is "rock solid."

As Kerry spoke inside, Joseph Puder was outside handing out flyers headlined, "The Iran Deal is Bad for America." He said Iran aspired to drive the United States out of the region and to spread nuclear capability to threatening regimes.

"The worst part of the deal is Obama's going to have a legacy of nuclear proliferation throughout the Middle East," said Puder, executive director of the Philadelphia chapter of Stand With Us, an Israel support group. "The whole region is going to go nuclear."

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Inquirer staff writer Jack Tomczuk contributed to this article.