New York Sen. Chuck Schumer Tuesday said if Congress rejects the Iran nuclear deal, the U.S. could force its allies and Iran back to the negotiating table through "secondary sanctions" on European businesses, prompting Secretary of State John Kerry to respond: "Are you kidding me?"

Schumer, the only Senate Democrat opposing the deal, spoke out for the second day against the agreement between six nations led by the United States and Iran. It would curb Iran's nuclear program in return for lifting sanctions. Schumer called it a deal with too many flaws and weaknesses.

Kerry and the White House have tried to play down the defection by Schumer, one of the most powerful lawmakers in Congress who is in line to be the Senate Democratic leader in 2017. But Kerry has objected to Schumer's arguments to reporters and the public.

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Like many opponents of the deal -- including most Republicans -- Schumer, a staunch supporter of Israel, said the U.S. can get a better agreement. Kerry told a July Senate hearing that is "fantasy."

Schumer's suggestion on how to restart negotiations if the U.S. backs out of a deal painstakingly crafted over two years, touched off a debate.

"It's not easy to go back to diplomacy," Schumer said at an unrelated event in Hauppauge. "But the U.S. has a good weapon there, which is what's called the secondary sanctions, which would say to HSBC, a big British bank . . . 'If you deal at all with Iran, you can't deal anywhere with the United States and we're a much bigger market.' "

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Asked by a reporter if that is realistic, Schumer said it was.

"How do you think we got them to the table, the other countries, in the first place?" Schumer asked. "It was the secondary sanctions."

But later at a Reuters Newsmaker event in Manhattan, Kerry said of that approach, "Are you kidding me?"

Kerry said, "The United States is going to start sanctioning our allies and their banks and their businesses because we walked away from a deal? And we're going to force them to do what we want them to do, even though they agreed to the deal we came to?"

He warned the consequences could be that the United States would lose its moral high ground and Israel could become isolated as European governments walk away from the sanctions. Schumer also said the United States could keep up pressure on Iran if European countries lifted their sanctions assuming the agreement was blocked by Congress. But Kerry said he and President Barack Obama have determined armed conflict will result without the deal.

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"We will have lost the support of the international community if we have to do a military action," Kerry told reporters Monday in Washington. "We would have been the people who turned away from the arrangement where [European governments] were willing to live by an agreement with huge restraints and transparency, accountability, inspectors, and we will not have the moral authority or the leverage."