WASHINGTON — If Iran sanctions legislation introduced Thursday were to pass Congressional hurdles, President Barack Obama would veto it, White House Spokesman Jay Carney told reporters. Speaking an hour after senators announced the bipartisan Nuclear Weapon Free Iran Act, Carney slammed the legislation, describing it as potentially “damaging and destructive to the diplomatic effort.”

Carney implied that lawmakers were out of step with American voters in proposing legislation that he said “will undermine our efforts to reach a diplomatic solution and greatly increase the chances for military action.”

“I think that there is overwhelming support in the country and in this congress for a diplomatic resolution to this conflict,” Carney said.

Characterizing the legislation as “unnecessary,” he said that “if it passed, the president would veto it.”

Carney said that he could not recall another instance in which the president would veto Democrat-sponsored legislation. The president, he added, has been “involved” in the effort to convince Democratic senators not to introduce the bill, and senior White House officials have made personal calls to leading senators in recent days – but to no avail.

Senators Robert Menendez (D-NJ) and Mark Kirk (R-IL) sparked a firestorm in Washington Thursday afternoon when they introduced the bill, which proposes additional sanctions on Iran should the regime violate the interim Joint Plan of Action agreed to in Geneva or fail to reach a final agreement with the six world powers.

Top administration officials, including Treasury Secretary Jack Lew, Secretary of State John Kerry and top US P5+1 negotiator Wendy Sherman, have made multiple trips to Capitol Hill to try to convince senators not to advance such legislation.

State Department Deputy Spokeswoman Marie Harf castigated the agreement in a prepared statement during a State Department briefing.

“We strongly oppose the action taken by these members of Congress,” she said, adding that “this legislation does not provide the president and negotiations team the flexibility needed to reach a diplomatic resolution with Iran.”

The bill’s sponsors, she complained, “have chosen to ignore the recommendations of our negotiators and the intelligence community.”

Harf warned that the additional sanctions could cast the United States as a bad faith negotiator. She emphasized that the text of the interim deal had included a commitment that no new sanctions would be imposed while the six-month interim period was underway.

In what she characterized as a worst-case scenario, Harf warned that the legislation could cause Iran to leave talks altogether.

“We don’t think it’s worth the risk of threatening negotiations for something that could be passed overnight if need be,” she complained. Kerry, said Harf, “has been very clear that if this falls apart we will be the first ones up to Congress asking for new sanctions. Anyone who follows this knows that Congress could do this in 24 hours.”

Despite vocal opposition from the Executive Branch, the bill found praise from the Anti-Defamation League and pro-Israel lobbying group AIPAC. The ADL said in a statement that legislation “makes clear to Iran the severe economic price it will pay if it does not negotiate seriously and plays for time” and, if passed, it would “ensure that Iran gives up its nuclear weapons program.” AIPAC issued an online petition calling on Americans to write to their representatives in Washington, urging them to back the bill which “provides guidance to the administration for the acceptable contours of a final agreement between Tehran and the P5+1.”