Israeli Prime Minister Benjamin Netanyahu tried to cancel a January briefing for U.S. Senators by his nation’s intelligence service that warned Congress could damage talks aimed at constraining Iran’s nuclear program, according to sources familiar with the events.

Tennessee Republican Bob Corker, the head of the Senate Foreign Relations Committee, had requested the Jan. 19 briefing for six of his colleagues traveling to Israel so that the intelligence agency, Mossad, could warn them that a Senate proposal might inadvertently collapse the talks. After Netanyahu’s office stripped the meeting from the trip schedule, Corker threatened to cut his own Israel trip short in protest.

Netanyahu relented after the personal intervention of Israeli ambassador to the U.S., Ron Dermer, and allowed the briefing to go forward, sources say. Attending were Corker, Republican Senators John McCain, Lindsey Graham and John Barrasso, Democratic Senators Tim Kaine and Joe Donnelly, and Independent Senator Angus King.

At issue was the fate of a Nov. 2013 agreement between Iran, the U.S. and five other international powers. That temporary agreement promised no new economic sanctions on Iran in exchange for a freeze of Iran’s nuclear program, new international inspections of Iran’s nuclear sites and the removal of nearly all medium-enriched uranium from Iran’s possession. Both sides have stuck to the interim deal while talks on a long-term deal to constrain the Iranian nuclear program have dragged out.

The controversial but popular bill proposed by Republican Mark Kirk and Democrat Robert Menendez would have imposed new sanctions on Iran if it didn’t agree by June 30 to a long-term deal. U.S. intelligence officials had concluded that the Kirk-Menendez bill risked collapsing the talks and taking with it the 16-month-old agreement, according to a report by Eli Lake and Josh Rogin of Bloomberg View. Corker wanted the Mossad briefing to bolster the U.S. assessment.

During the Mossad briefing, the agency’s chief, Tamir Pardo, warned that the Kirk-Menendez bill would be like “throwing a grenade” into the U.S.-Iran diplomatic process. After some of the contents of the briefing were first reported by Bloomberg View, Pardo released a statement saying he had used the phrase not to oppose new sanctions, but “as a metaphor” to describe the effect derailing current talks might have.

A spokesman for Netanyahu declined to say why the Prime Minister acted to prevent the Senators from receiving the briefing from Pardo. Since the Mossad briefing, Corker has rallied support for an alternative measure to replace the Kirk-Menendez proposal, support for which has faded. Corker’s bill, which has broad support and potentially could receive enough votes for a veto-proof majority, would only impose new sanctions if Iran walked away from the Nov. 2013 agreement.

U.S. and Iranian officials are entering a tense phase of negotiations in Switzerland this week as they attempt to reach a political deal to extend and expand the Nov. 2013 agreement for at least 10 years. As the challenges of reaching the longer-term deal have increased, some in the U.S. are trying to ensure the interim agreement that has frozen the Iranian program isn’t undermined in the process.

Some members of the Senate oppose the ongoing talks with Iran. Freshman Republican Senator Tom Cotton last week issued an open letter with 46 other GOP Senators warning the Iranian leadership that Congress could reverse parts of any deal the talks produce. Corker did not sign that letter; his bill provides for partial Congressional approval of a deal.

Cotton has said that rather than negotiate with Iran, the U.S. should adopt a policy of regime change and should arm Israel with bombers and bunker busting bombs with which it could attack Iranian nuclear sites. Authorities in both parties, including Obama’s first Defense Secretary Robert Gates, have worried that an Israeli attack could draw the U.S. into a military confrontation with Iran on unfavorable terms.

Supporters of Kirk-Menendez argue it would increase pressure on Iran to make concessions that would more effectively limit its ability to get a nuclear weapon. Republicans are concerned that the Obama administration is too eager to do a long-term deal with Iran and is making too many concessions in the current talks. Secretary of State John Kerry arrives for talks in Geneva Sunday ahead of a self-imposed Mar. 24 deadline for the political framework for a long-term deal. Final terms of a comprehensive agreement would not be worked out before June.

Netanyahu is seeking re-election in a tough vote Tuesday, with his Likud Party trailing his strongest competitor, Zionist Union, by four points in recent polls.

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