This article is from the archive of our partner National Journal

Senate Foreign Relations Chairman Bob Corker ushered his Iran-nuclear-approval bill through the tumultuous hearing process. He managed to hold together a fragile coalition of Republicans and Democrats to pass it out of committee. But now Corker and his Democratic counterpart Ben Cardin are attempting their most difficult feat yet: guiding the Iran bill on Senate floor as it weathers presidential politics.

In the Senate, the tension between 2016 hopefuls, such as Marco Rubio and Ted Cruz, and the chamber's pragmatists has finally come to a head.

On Wednesday, Rubio delivered an impassioned speech on the floor of the Senate, defending a handful of amendments he has proposed that other senators have accused of being "poison pills" with the potential to kill the delicately-worded Iran bill. One such amendment would require that the Iranians "publicly accept Israel's right to exist as a Jewish state." Another would hold the White House to the framework it initially laid out as it seeks to negotiate with Iran.

But while Rubio has been accused by fellow senators of grandstanding at an inopportune time, he had a message for senators like Corker and Cardin who have appealed to their colleagues in conference meetings, on the floor, and privately.

(RELATED: Can Republicans Have it All on Iran?)

"If you don't want to vote on things, don't run for the Senate," Rubio said on the floor Wednesday afternoon. "Everyone who runs for office knows that what we are called to do is vote on issues that sometimes we're uncomfortable on. There is a microphone here on your desk. Come here and explain to the world why you are voting against a deal that requires Israel to have a right to exist."

What makes Rubio's amendments so contentious for rank-and-file members is not their substance. Many senators on Capitol Hill agree with the policies he's pushing. Most senators want Israel to be recognized. But many elder senators have reminded their junior, ambitious colleagues that this is the body's only chance to weigh in on a deal cut between President Obama and Iran's leaders. It is for that reason that interest groups such as AIPAC have called on senators to back off on efforts to push for tough amendments.

After Rubio spoke, Cardin took to the floor and gave his own fiery defense of the committee's bill as it stood. He said Rubio's amendment would have "unintended consequences" that would make it "virtually impossible" for the president to sign the bill. Without a White House signature, Cardin argued that Congress would likely lose its power to disapprove of a bad nuclear deal.

"It is clear that if that amendment got on the bill it would mean one of three things," Cardin told reporters earlier in the day. "It will either mean the bill will fail and not get to the finish line; it means that we cannot negotiate an agreement with Iran so we become isolated in America in our efforts to get a negotiated settlement; or at a minimum it puts Iran in a stronger position in the negotiations."

(RELATED: Tom Cotton Started a Twitter Spat With Iran's Foreign Minister)

Senators on both side of the aisle are fuming at the prospects of losing their opportunity to weigh in on a nuclear deal because of their colleagues' presidential prospects.

During a Republican lunch Wednesday, a senator recounted that Corker and Cruz engaged in a spirited back and forth on Cruz's own amendment, which would require Congress to approve of any nuclear deal sent over from the White House with a majority. Cruz's amendment would allow a straight party vote on an Iran nuclear deal in Congress, which Obama is sure to not sign.

"This is playing out the way I thought it would," said Sen. Lindsey Graham, who also is considering a White House run. "You got people who don't know what they are doing, very immature, very focused on themselves, coming to the floor and coming up with legislative proposals that are just pie in the sky."

Graham added that he supports Rubio's proposals in principle, but will not vote for anything he thinks could stand in the way of a bill to get a vote on an Iranian nuclear deal. 

(RELATED: A Test for the Iran Deal -- And the 2016 Contenders)

"Any amendment that doesn't advance that cause or can break apart that bipartisan coalition and take us down a sidetrack is a blessing to the Ayatollahs," Graham said. 

In both cases, Rubio and Cruz have an opportunity to use the amendment process to boost their support in conservative circles. But that political gain may come at a cost for the rest of the Senate. The instance serves as yet another reminder how the campaign trail is manifesting itself on the floor of the Senate.

Even Corker acknowledged the senators' split priorities on the floor Tuesday, alluding to the fact that many of the 2016 contenders were "traveling around the country, focused on other things at present."

There is a deep concern among many senators that too many amendments could undercut the Senate's ability to have any say at all over the Iran negotiation.

"This is our only option for the Senate to exercise what I believe is our constitutional duty to review and vote on this likely agreement," said GOP Sen. Susan Collins, who said she agrees that the bill could be stronger.

The reality is that in order to pass the Iran bill and in order for it to maintain broad political support without undermining nuclear negotiations, senators will be forced to chose between blowing up this carefully crafted bill or voting on principle. Being on record against freeing U.S. prisoners or denying Israel the recognition senators believe it deserves is an uncomfortable place to be.

This article is from the archive of our partner National Journal.

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