This week could decide the fate of a bipartisan bill allowing the U.S. Congress to review and vote on any final nuclear accord with Iran. The Senate can either vote on the original bill, or allow amendments making additional demands of Iran, which would trigger a presidential veto and could doom the entire legislative effort.
If enacted in its current form, the bill would freeze U.S. sanctions relief on Iran for 30 days while Congress reviews a final nuclear pact with Tehran. It has the strong backing of Senate Majority Leader Mitch McConnell.
"These sanctions are a big reason why America was able to bring Iran to the (negotiating) table in the first place. We should not be giving up that leverage now without the American people, through members of Congress they elect, having a chance to weigh in. Quite simple," he said.
But some of the very Republicans McConnell leads are putting him in a tough position. They are trying to force votes on amendments on side issues to the nuclear talks, like compelling Tehran to recognize Israel's right to exist.
The so-called "poison pill" amendments could derail the bill, but Senator Tom Cotton is unapologetic.
"We need to vote. If you do not want to vote, you should not have come to the Senate. If you are in the Senate and you do not want to vote, you should leave." Cotton added "We are talking about a nuclear-armed Iran, the most dangerous threat to our national security."
The authors of the bipartisan bill to give Congress major oversight role in a potential deal to limit Iran's nuclear program, Senators Bob Corker and Ben Cardin, have pleaded for restraint.
Republican Corker is chairman of the Senate Foreign Relations Committee, which unanimously approved the bill last month. Cardin, the committee's top Democrat said "this is an extremely important issue. It has got to rise above our individual desires so that, collectively, we can achieve something for the American people."
President Barack Obama says the chance to verifiably limit Iran's nuclear ambitions must not be squandered by airing the United States' many grievances with Tehran.
White House spokesman Josh Earnest said the U.S. does not anticipate that the negotiations will resolve the long list of concerns Washington has with Iranian behavior."We know that it would be even more difficult to resolve our concerns about Iran's destabilizing activities if Iran had a nuclear weapon," he added.
McConnell can either allow votes on polarizing amendments, or move to end floor debate and bring the unaltered bill up for a final vote.
Iran and the group that includes the U.S., Britain, China, France, Russia and Germany have set a June 30 deadline for finalizing a deal after agreeing to a framework earlier this month.
U.S. Secretary of State John Kerry and his Iranian counterpart Mohammad Javad Zarif met last week in New York for private talks. Before the meeting, Kerry told a U.N. conference on nuclear non-proliferation that a deal would make the world safer.
In exchange for curtailing its nuclear activity, Iran would get relief from sanctions that have hurt its economy. Iran has long insisted its nuclear program is peaceful with a focus on uses such as power generation and medical research, not trying to build bombs.