Secretary of State John Kerry sparred Tuesday with the lone Democratic senator to publicly oppose last month’s historic Iran nuclear deal, saying there was no way the U.S. could prevent American allies from doing business with Tehran if Congress were to reject the agreement.

Speaking across town in New York, Sen. Chuck Schumer disagreed and suggested Washington still could force the world into isolating the Iranians until they make deeper nuclear concessions.

The dispute goes to the heart of the questions that American lawmakers are considering as they prepare to vote on the nuclear accord.

If they were to shelve the deal — and override an expected presidential veto — they could severely complicate the Obama administration’s ability to honor its commitments to roll back economic sanctions on Iran. In exchange, Iran has agreed to a decade of tough restrictions on Iran’s nuclear program and a far more intrusive inspections regime.

Republicans are almost universally opposed at this point.

Addressing a Reuters Newsmaker event in New York, Kerry took aim at those in Congress who say a better deal could still be reached. That argument would entail the U.S. maintaining or increasing pressure on Iran by threatening foreign governments and businesses for trading with Tehran or buying Iranian oil, a strategy that both President Barack Obama and Republicans credit with drawing Iran into serious nuclear negotiations two years ago.

Now that the pact has been finalized, Kerry said such a heavy-handed approach was an option no longer.

“Are you kidding me?” he asked the crowd. “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?”

Kerry warned of severe consequences for pursuing such an approach after the agreement has been accepted by Iran and fellow negotiating countries Britain, China, France, Germany and Russia — and endorsed by all 15 members of the U.N. Security Council.

He said that European governments could walk away from the U.S.-led sanctions strategy against Russia, that the United States and Israel would have no support for military action against Iran, if such action were necessary, and that the U.S. dollar would lose its status as the reserve currency of the world.

The top American diplomat also challenged those who have criticized the length of the deal’s restrictions on Iranian enrichment of material that can be used in nuclear warheads and other elements of its program.

He suggested it was illegitimate to worry that Iran would be a “nuclear threshold nation” in 15 years or 20 years, because it already is one today. “They became that while we had a policy of no enrichment,” he said, referencing the continued demand of Republicans and Israeli Prime Minister Benjamin Netanyahu.

If Congress were to vote down the deal, he said, the U.S. would lose the moral high ground.

“We will have left Iran free to go do its program, without restraints, without inspections, without knocking down its stockpile, without knowing what they’re doing,” he said.

Echoing Kerry’s case, 36 retired generals and admirals released an open letter on Tuesday urging Congress to back the deal.

“Military action would be less effective than the deal, assuming it is fully implemented,” the letter said. “If the Iranians cheat, our advanced technology, intelligence and the inspections will reveal it, and U.S. military options remain on the table. And if the deal is rejected by America, the Iranians could have a nuclear weapon within a year. The choice is that stark.”

Kerry’s comments came shortly after Schumer told reporters why he decided last week to oppose the agreement.

Even if the U.S. were to back away and other countries lifted their sanctions, he contended that “powerful” secondary sanctions would prevent many foreign governments from doing business with Iran and force them into new negotiations with Iran. He cited the French oil company Total as one that would suffer if it dealt with Iran, because then it would be locked out from the U.S. market.

“We have that powerful tool, and if used, I think that’s a better, better chance in a very difficult world than an agreement that is so totally flawed,” Schumer said.

Schumer is in line to be the Democrats’ next leader in the Senate and his defection was seen as a significant blow to Obama’s effort to sell the Iran deal to Congress. Since then, however, a handful of Senate Democrats and several House Democrats have announced their support.

Schumer was asked by reporters if he would lobby colleagues to vote with him.

“Certainly, I’m going to try to persuade my colleagues that my viewpoint is right, but anyone who thinks you can force somebody to vote with you in the Senate doesn’t understand the Senate,” he said. “This is a vote of conscience. It was a vote of conscience for me. It will be a vote of conscience for my colleagues.”

Photo Credit: Shmuel Lenchevsy

(AP)