The expected end to the debate over a multilateral nuclear agreement with Iran that has consumed Congress for weeks took another unexpected turn shortly before senators came to the floor for a vote Tuesday.

For the second time in less than a week, Senate Democratic supporters of the agreement between the United States, partner nations and Iran blocked a resolution of disapproval from advancing in a 56 to 42 vote, an anticipated outcome that underscored the reality that President Obama’s landmark foreign policy achievement will stand. Opponents of the deal needed 60 votes to move forward.

But just before the voting began, Senate Majority Leader Mitch McConnell (R-Ky.) said his chamber would continue to vote on related issues later this week.

“If cloture is not invoked, I will file an amendment that would prevent the president from lifting sanctions until Iran meets two simple benchmarks: It must formally recognize Israel’s right to exist and it must release the American citizens being held in Iranian custody,” McConnell said on the Senate floor. McConnell’s office said a cloture vote on his amendment would take place Thursday.

Senate Minority Leader Harry Reid (D-Nev.) called the move a “charade.”

“I wonder if the Republican leaders bother to look at the calendar lately,” Reid said in response to the latest chapter in the Iran deal saga. “This is not an intelligent debate.”

The amendment, which McConnell filed this evening, will set up a politically difficult vote later this week for several Democratic senators who decided to support the deal even after expressing skepticism. A vote cast against the proposed amendment would be ideal campaign fodder for Republicans seeking to unseat vulnerable Democrats like Sen. Michael Bennet (D-Colo.).

The accord would reduce economic sanctions levied against the long-estranged Gulf state in exchange for an end to its pursuit of nuclear weapons.

Republicans in Congress have relentlessly criticized the administration’s efforts, and Washington spent the August recess watching the moves of congressional Democrats to determine whether Obama’s signature international accomplishment could survive.

Under the terms of a law written by Sens. Bob Corker (R-Tenn.) and Ben Cardin (D-Md.), passed earlier this year, the agreement would stand unless Congress passed a resolution of disapproval before the end of a 60-day window. Because Obama would be able to veto such a motion, the White House only needed to convince one-third of the members in either the Senate or House to preserve the accord.

Democratic support for the accord gradually built throughout the summer, until it became clear shortly before lawmakers return to the Capitol in September that Reid and Sen. Dick Durbin (D-Ill.), a close Obama ally, had mustered the votes necessary to not only sustain a presidential veto, but also block a resolution of disapproval from ever reaching the president’s desk with a filibuster.

In the end four Democrats broke ranks with the White House: Sens. Bob Menendez (D-N.J.), Joe Manchin (D-W.Va.), Chuck Schumer (D-N.Y.) and Cardin.

Last week, the House Republican leaders reversed course on holding a vote on its own resolution of disapproval after a number of Republicans objected to the fact that Congress has not been presented with the details of two agreements negotiated between Iran and the U.N.’s International Atomic Energy Agency.

They argued that the administration’s failure to make those documents available violated the requirements of Corker-Cardin, meaning that the 60-day clock had yet to start.

The White House and Democrats quickly rejected the argument, and even GOP leaders in the Senate – while expressing concerns about the side agreements – acknowledged that crying foul over the review window was an exercise in futility.

Nonetheless, House Speaker John Boehner (R-Ohio) and his leadership team held three votes on the deal to allow members to register their concerns. One of those was a resolution approving, rather than disapproving, the nuclear accord. As intended, that measure was easily voted down, 269 to 162.

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