The Senate on Thursday is continuing to debate an Iran bill after the body rejected a Republican amendment a day earlier that would have jeopardized White House support for the legislation. The GOP addition would have required President Barack Obama to certify that Iran has not been involved in terrorism as a condition for signing a nuclear agreement with that country, a high bar that could have threatened the viability of the deal with Iran that’s in the works. The amendment’s defeat was a significant victory for proponents of the bipartisan bill, which would give Congress the ability to weigh in on that nuclear deal if it is reached. READ: First on CNN: U.S.-flagged ship intercepted by Iranian patrol They had argued the amendment was a “poison pill” because the certification requirement was unworkable and could have led the President to veto the bill. The measure fell 15 votes shy of the 60 it needed to pass. It was the second controversial amendment to be voted down in two days, a strong sign the underlying bill is headed towards passage and a presidential signature. Tensions rose on the Senate floor earlier when the top Democrat on the Foreign Relations Committee, Sen. Ben Cardin of Maryland – who worked out the compromise version of the bill that Obama has agreed to approve after opposing an earlier draft – charged that a separate amendment offered by Republican Florida Sen. Marco Rubio was also a poison pill that would kill the bill. At issue was a proposal from the 2016 presidential candidate to require Iran to recognize Israel’s right to exist as a condition for having sanctions lifted on the Islamic state. Rubio complained that without his amendment the emerging deal with Iran “would do nothing about Iran’s self-proclaimed support, at the highest levels of government, for the destruction of Israel.” Recognizing Israel’s right to exist is anathema to Iran’s leaders. Cardin’s voice rose with emotion as he pointed his finger while blasting the amendment. He argued that a multi-national deal preventing Iran from getting nuclear weapons would be more important to Israel than a potentially disruptive amendment requiring Iran to recognize its right to exist. “That’s what Israel needs and that’s what we’re trying to get.” Cardin said. “Yes, it’s a poison pill. Yes, it will defeat this bill.” Cardin blocked Rubio’s request for an immediate vote on the amendment, but Rubio said he hoped to get a vote on it as the debate on the bill continues. The heightened emotions on Capitol Hill didn’t only stem from what was happening in the legislative chamber. Iran’s seizure this week of a Marshall Islands-flagged merchant ship and its interception last week of a U.S.-flagged ship, both in Iranian territorial waters in the Strait of Hormuz, intensified lawmakers’ concerns about U.S. deal-making with Tehran. Republican senators, already skeptical of the administration’s ability to cut a robust agreement curbing Tehran’s nuclear program, were frustrated with Iran’s hostile moves and said they were indicative of the aggressive and untrustworthy Iranian regime. Sen. John McCain, R-Arizona, the chairman of the Armed Services Committee, said Wednesday that the moves were “dramatically escalatory” and “a gross violation of international law.” Sen. Jim Risch, R-Idaho, a member of the intelligence committee, described the Iranian behavior as a response to the Obama administration’s decision last week to send warships to monitor and deter an Iranian convoy the U.S. was concerned might supply arms to rebels in Yemen who have deposed the Western-backed government. “What did Iran do?” he asked. “All they did was push the envelope and cause more aggravation.” READ: Rubio, Kirk: No nuclear deal unless Iran releases reporter Some Republicans tied the moves to the incipient deal and suggested it could further hurt already shaky congressional support. “You would think the Iranians would be on their best behavior right now as the administration attempts to complete the deal and convince those of us who have expressed concerns and who are skeptical of it that the Iranians will live up to the good intentions,” said Sen. Susan Collins, R-Maine, another member of the intelligence committee. Sen. Ron Johnson, R-Wisconsin, a member of the Foreign Relations Committee who strongly opposes the emerging nuclear deal with Iran, put it more strongly. He said the incidents should be “the final nail in the coffin in terms of negotiations.” But despite the outrage, senators said they didn’t expect Iran’s recent naval actions to have much impact on the Senate bill itself. “I don’t’ think people are going to try to tie that incident to this bill, but I do think that it’s again another signal of the kind of country we’re dealing with,” said Sen. Bob Corker, R-Tennessee, the chairman of the Foreign Relations Committee and co-author of the Iran bill, who worked with Cardin on the compromise version and has tried to roll back some of his fellow Republicans’ most aggressive moves concerning the measure. “Everybody here is so disgusted with Iran it’s pretty hard to raise the disgust level any higher,” Risch added. A final vote on the Iran bill is expected as early as next Tuesday.