A key enforcement tool of the nuclear deal with Iran is the idea that international sanctions would quickly "snap back" into place if Tehran violates any of its terms.

The deal signed July 14 in Vienna allows international sanctions to be reimposed anytime in the next 10 years if one of the parties to the agreement — the United States, Britain, France, Germany, Russia or China — brings a violation to the U.N. Security Council. Only a vote of the council, which the United States can veto, can block the sanctions from being reimposed.

But Iran has a "snap back" mechanism of its own: If Iranian officials feel its international partners reimposed sanctions unfairly, they can unfreeze their country's nuclear program and resume enriching uranium without limits. It's a key element in concerns that Iran was developing a nuclear weapon.

"The sanctions regime, the economic sanctions regime, is being dismantled while Iran's nuclear program is not," sanctions expert Mark Dubowitz of the Foundation for the Defense of Democracies told the House Foreign Affairs Committee on Thursday, saying that Iran will use the hundreds of billions of dollars in relief to immunize its economy against economic pressure, leaving military force as the only viable option to prevent Iranian violations.

"This provides an Iranian insurance policy even in the case of severe violations and certainly in the case of small- to medium-sized violations. It gives Iran a powerful tool to stonewall the [International Atomic Energy Agency], undermine the dispute resolution mechanisms and deter U.N., E.U. and U.S. snapbacks," he said.

The loss of U.S. leverage from the removal of the tough sanctions that have brought Iran's economy to its knees is a major concern of lawmakers evaluating the deal. Congress has until Sept. 17 to either approve the deal, disapprove of it or do nothing. Only a resolution of disapproval that can survive a presidential veto will prevent President Obama from waiving sanctions written into U.S. law.

A further concern is the fact that those sanctions expire in 2016, well ahead of the nuclear deal itself. At a Senate Foreign Relations Committee hearing Thursday, Democrat Bob Menendez of New Jersey, a co-author of the law, pressed Treasury Secretary Jack Lew on whether the administration would support renewing the sanctions.

"My point is this. If you're going to snap back, you've got to snap back to something," Menendez said in a contentious exchange, interrupting Lew to note that if the existing sanctions expire "there is nothing in that context to snap back to."

"Right now the sanctions remain in effect," Lew said, and continued: "It's premature to talk about extending a law ..." But Menendez cut him off.

"I don't understand how we ultimately have a credible belief that snapback means something if in fact you're not going to be able to have those sanctions in place," he said.

Later, Lew told Sen. Cory Gardner, R-Colo., that "the snapback provisions are extremely powerful with or without the Iran Sanctions Act."