J. Scott Applewhite / AP

Updated May 7 2015, 4:15 p.m.

The Senate on Thursday afternoon overwhelmingly approved legislation allowing Congress to review a potential nuclear agreement with Iran. The vote was 98-1, as lawmakers came together across party lines to assert their role in a key foreign-policy decision.

The only senator to oppose the legislation was Tom Cotton, the freshman Republican from Arkansas who has campaigned against the Obama administration’s negotiations with Iran and was blocked from offering amendments to the bill. The House is expected to take up—and likely pass—the measure in the coming weeks.

Passage of the legislation still won’t make it easy for Congress to reject the Iran deal. Under a compromise worked out by Senators Bob Corker, the Tennessee Republican, and Benjamin Cardin, a Maryland Democrat, lawmakers would have 30 days to approve, disapprove, or take no action on a final nuclear agreement. If Congress failed to act, the deal would take effect. And any vote of disapproval would be subject to a presidential veto, meaning that President Obama would need the support of just 34 senators to sustain an agreement. The White House dropped its opposition to the bill last month, and Obama is expected to sign it.

Updated May 6, 2015, 6:17 a.m.

Last week, Senate Majority Leader Mitch McConnell turned to the members of his caucus with their eyes on the White House, and asked, "You think running for president is hard?" He didn't have to complete the thought, but it would have been: Try running the Senate.

The Kentucky Republican has been tested from the start, but the debate over legislation to give Congress a role in reviewing the Obama administration's potential nuclear agreement with Iran is showing just how complex his job can be. The bill is a rare proposal with broad bipartisan support, and its passage on Thursday marks a notable victory for lawmakers in their long-running tug-of-war with the White House over the proper balance of powers. McConnell's problem was not with Democrats but with Republican Senators Tom Cotton and Marco Rubio, among others, who wanted votes on changes to the bill that threaten the hard-fought consensus its backers have built. Call it death by amendment.

Rather than indulge Cotton and Rubio, McConnell moved to end debate on the bill, cutting off the opportunity to offer amendments and expediting its passage later this week or next. It's a move that Harry Reid, the Nevada Democrat, perfected during his time as majority leader. Yet that's exactly the kind of top-down, heavy-handed maneuver that McConnell pledged to curtail if he was in charge. That he did it to prevent members of his own party from poisoning a bill makes it all the more awkward.

Just four months into his Senate career, Cotton has made a name for himself with his opposition to the Iran deal, and he tried his own procedural gambit last week to force a vote on his amendment to compel Tehran to shut down all its nuclear facilities—a certain deal-breaker. Rubio also proposed a number of amendments, but the most controversial—at least in the context of the Senate debate—sought to make Iran's recognition of Israel a necessary condition of any final nuclear accord. The vote would have been politically difficult, if not impossible, for senators of both parties to oppose, but its inclusion could have torpedoed the bill, as its supporters had already pledged to block efforts to undermine the administration's negotiations with Iran.

McConnell's strong-arming won't necessarily make it any easier to finalize a deal with Iran. Under the compromise struck by Senators Bob Corker and Ben Cardin, Congress's ability to reject a nuclear agreement would have been limited, so much so that the White House dropped its objection to the interference from Capitol Hill. Had Cotton or Rubio succeeded, the most likely casualty would have been that opportunity for congressional review, as opposed to the negotiations themselves.

The greater significance of McConnell's decision is his willingness to turn aside a pair of rising stars in his caucus. As McConnell has learned from Speaker John Boehner, managing the more confrontational conservatives is a constant challenge for any Republican leader who hopes to pass legislation that has a chance of becoming law. Cotton has already made the most out of his aggressive moves in the Iran debate, so it's not particularly surprising that McConnell is now telling him, "Enough." Yet the Senate leader's unique challenge over the next year will be handling the four declared presidential candidates among his members (Rubio, Ted Cruz, Rand Paul, and Lindsey Graham), all of whom will seek, at one time or another, to use the Senate as a tool to advance their campaigns. McConnell is nominally backing Paul, his home state's favorite son, but his move against Rubio can be read as a signal to all four hopefuls that he is not ready to relinquish the Senate floor to their political designs.

On the whole, McConnell has had an up-and-down start to his tenure as majority leader. He waged a rather hopeless battle over immigration policy in a funding bill for the Department of Homeland Security, and he spent a month on the Republicans' first major bill—to approve the Keystone pipeline—only to watch Obama swiftly veto it. Yet McConnell has also presided over an undeniably more productive Congress so far; the Senate has approved bipartisan legislation to overhaul Medicare payments and combat human trafficking, confirmed two Obama Cabinet appointees (one after needless delay, however), and passed a Republican budget.

McConnell has also made progress on what he said would be his top priority as majority leader: "Restoring the Senate" as a functioning legislative body—one that debates, amends, and ultimately passes bills rather than bottling them up or suffocating them with filibusters. And some Democrats have acknowledged there have been improvements in the legislative climate this year, even if they come in fits and starts, and even if the Senate occasionally reverts to old habits. The Iran bill is a prime example. Supporters banded together to defeat a pair of "poison pill" amendments last week before Cotton and Rubio got in the way. McConnell spent a few days trying to find a compromise. But when that didn't pan out, he shut down debate to save the bill, and with the presidential campaign heating up, this probably won't be the last time he has to sacrifice process for substance.

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