WASHINGTON – Democratic leadership in the US Senate has grown increasingly reluctant to act on a bill that would threaten Tehran with harsh new sanctions should it fail to reach a final agreement on its nuclear program.

US President Barack Obama made a plea for time to explore diplomacy in his annual State of the Union address to a joint session of Congress on Tuesday night.

The Nuclear Weapon Free Iran Act of 2013 still has 59 co-sponsors, including 19 Democrats, publicly supporting the bill. But Senate Majority Leader Harry Reid has given no indication he has any plans to bring the legislation to a vote, as his party colleagues begin to openly express skepticism on its timing.

“I am strongly supporting the bill, but I think a vote is unnecessary right now as long as there’s visible and meaningful progress,” Sen. Richard Blumenthal (D-Connecticut) told Agence France-Presse this week. Blumenthal is a co-sponsor on the legislation.

The point of the bill, as intended by its authors, Senate Foreign Relations Committee chairman Robert Menendez (D-New Jersey) and Sen. Mark Kirk (R-Illinois), is to hold the negotiations process to account with the specter of consequential failure: Should six months pass without a comprehensive agreement ending the nuclear impasse, the bill would automatically trigger new sanctions tools on the Islamic Republic, pending presidential approval.

Proponents of the bill consider it negative reinforcement for Iran to engage meaningfully in negotiations over its nuclear program with the P5+1— the US, United Kingdom, France, Russia, China and Germany.

But critics of the legislation, led by the Obama administration, warn that its passage could undermine the diplomatic process, and perhaps derail it completely.

In his fifth State of the Union address, Obama promised to veto the measure should it reach his desk.

“If Iran’s leaders do not seize this opportunity, then I will be the first to call for more sanctions, and stand ready to exercise all options to make sure Iran does not build a nuclear weapon,” he said. “But if Iran’s leaders do seize the chance, then Iran could take an important step to rejoin the community of nations, and we will have resolved one of the leading security challenges of our time without the risks of war.

“For the sake of our national security, we must give diplomacy a chance to succeed,” the US president said.

Under the Menendez-Kirk bill, no new sanctions would be imposed in the six-month interim. But Iran has stated that the bill nevertheless violates the Joint Plan of Action— the short-term deal reached in November between the P5+1 and Iran pausing its nuclear program in exchange for sanctions relief.

In that international agreement, the president agreed that “the US administration, acting consistent with the respective roles of the president and the Congress, will refrain from imposing new nuclear-related sanctions.”

Sen. Chris Coons (D-Delaware) said this week that “now is not the time for a vote on an Iran sanctions bill,” after the president urged Congress to hold off on the bill in the Tuesday address.

Kirk has been bullish on the bill since the president’s address.

Menendez has not commented on the matter. The Democratic leader on foreign policy matters will preside over a committee hearing on Iran next Tuesday.

Republican Senate aides warned that, should Democratic leaders tie up the legislative process, GOP members will be left with “no choice” but to resort to partisan procedure.

Meanwhile, Sen. Jeanne Shaheen (D-New Hampshire) wrote a letter to European Union foreign policy chief Catherine Ashton, expressing concern that EU trade delegations were opening up channels with the Islamic Republic, possibly in violation of their own sanctions regime.

“Delegations to Iran, including those from Europe, are premature and represent a step in the wrong direction as P5+1 negotiators work toward a comprehensive agreement that fully addresses the world’s concerns over the Iranian nuclear program,” Shaheen wrote in the letter, obtained by Politico.

“Given the importance of continued US-EU cooperation on Iran, I am deeply troubled by recent reports of EU member states sending or preparing to send extensive government and trade delegations to Iran,” she wrote.

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