WASHINGTON — Fresh from a spirited defense of the Iranian nuclear deal on Capitol Hill, Secretary of State John Kerry is selling the accord to top U.S. foreign policy leaders while telling lawmakers if they don’t back the deal, Washington will have “squandered the best chance” it has to peacefully solve the nuclear standoff with Tehran.

Kerry is to speak about the deal today at the Council on Foreign Relations in New York City and then meet with leaders from the American Jewish Committee and the Conference of Presidents of Major American Jewish Organizations. Some Jewish groups have vehemently opposed the deal with Iran, which has pledged to destroy Israel.

Kerry has been front-and-center of the Obama administration’s campaign to sell the deal to skeptical lawmakers. Congress has started a 60-day review of the agreement, which lifts economic sanctions against Iran if it curbs its nuclear program’s capacity to build a nuclear weapon.

Slowly, a number of congressional Democrats have lined up behind the deal.

Rep. Sam Farr, D-Calif., said he supports the agreement and described the upcoming vote in Congress as a “vote on something that will avoid war.”

At a Senate Foreign Relations Committee hearing Thursday, Kerry said it is “fantasy plain and simple” to claim President Barack Obama failed to insist on enough restraints on Iran’s nuclear program before agreeing to lift economic sanctions long in place.

“So what’s your plan? ... Totally go to war?” he challenged lawmakers.

“If the U.S., after laboriously negotiating this multilateral agreement with five other partners, were to walk away from those partners, we’re on our own,” Kerry said. “Our partners will not walk away with us. Instead, they will walk away from the tough multilateral sanctions regime that they’ve helped to put in place, and we will have squandered the best chance we have to solve this problem through peaceful means.”

Republicans were unpersuaded.

“I believe that you have crossed a new threshold in U.S. foreign policy where now it is the policy of the United States to enable a state-sponsor of terror to obtain an industrialized nuclear development program,” said the committee chairman, Sen. Bob Corker, R-Tenn.

Given the political calculus, the Senate hearing wasn’t so much an attempt by Kerry to persuade Republicans to support the plan as it was an opportunity to reassure Democrats.

And after four days of reading and mulling the more than 150-page agreement the U.S. and five other world powers negotiated with Iran, more Democrats are poised to publicly embrace it, according to a Democratic aide.