WASHINGTON -- 76 Senators signed a letter sent to President Barack Obama on Friday calling on the White House to toughen sanctions on Iran, despite the election of Hassan Rouhani, The Jerusalem Post has learned.In the letter, the large Senate group tells the president that time for diplomacy is quickly running out."Iran has used negotiations in the past to stall for time," the senators warn the president, noting Rouhani's former role as the regime's nuclear negotiator. "And in any event, Khamenei is the ultimate decision-maker for Iran's nuclear program." "Mr. President," the letter continues, "we urge you to bring a renewed sense of urgency to the process. We need to understand quickly whether Tehran is at last ready to negotiate seriously. Iran needs to understand that the time for diplomacy is nearing its end."The letter was sent to the president on the last day of the Senate session before summer recess— two days before Rouhani's inauguration in Iran— and was penned on July 30th, just a day before the House of Representatives voted overwhelmingly, 400-20, in favor of a punishing new sanctions package targeting what remains of Iran's oil sector.Earning significant bipartisan backing in just four days, the letter signals strong Senate support for the House bill, which aims to bring Iranian oil exports essentially down to zero by no longer providing exemptions to purchasing companies in allied countries.The senators assert that the US "must toughen sanctions and reinforce the credibility of our option to use military force at the same time as we fully explore a diplomatic solution to our dispute with Iran."Senators Robert Menendez, a Democrat and chairman of the Senate Foreign Relations Committee, and Lindsey Graham, a Republican member of the Senate Armed Services Committee, spearheaded the letter. Congressional sources tell the Post that the two-month window while Congress is out for summer, through an anticipated vote in late September on a Senate sanctions bill, will be pivotal for both the White House and the Iranian regime.The message: Rouhani must prove his seriousness in this brief window, or else punishing new sanctions are coming, sources say."We share your conviction that Iran must not be allowed to have a nuclear weapon," it concludes, adding, "we want you to know that you will have our support in doing all you can to resolve on an urgent basis this most pressing challenge to international security."The White House told the Post this week that it awaits talks at the "earliest opportunity" following Rouhani's inauguration on Sunday, adding that it has "some concerns over the specific contents of the legislation" but that it expects to work with Congress in the melding process."Following his inauguration, we hope President-elect Rouhani and the Iranian government will engage substantively with the international community to reach a diplomatic solution," National Security Council spokesperson Bernadette Meehan said. "We and our international partners remain ready to meet at the earliest opportunity once Iran is prepared to do so."