Ernest Moniz, U.S. secretary of energy, center, returned to the Capitol Wednesday for meetings with senators before their five-week break. Photographer: Andrew Harrer/Bloomberg

Senate leaders agreed Wednesday to shutter the chamber for the annual end-of-summer break and set up the pivotal debate on President Obama's nuclear pact with Iran after Labor Day, leaving just a handful of days to consider the proposal.

Setting aside cyber-security legislation for now, senators joined their compatriots in the House, who left last week for 40-day break until after Labor Day, when the lower chamber will also take up the Iran proposal.

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Under legislation approved earlier this review, Congress received a 60-day window to review the pact that Obama's senior diplomats reached with their Iranian counterparts in mid-July. Both chambers now have until Sept. 17 to act, or else Obama can fully implement the provisions of the deal, including lifting congressionally-imposed sanctions against the regime in Tehran.

A majority of lawmakers in the House and Senate are all but certain to oppose the plan, arguing that the inspection process was insufficient for assuring Iran doesn't secretly continue to push for weapons usage of its nuclear program. That's left the focus on whether Obama can pull enough votes, almost entirely from the Democratic side of the aisle, to allow him to veto the resolution and fully enact the deal.

Lobbying intensified this week ahead of the break. Israel's ambassador to the United States, Ron Dermer, paced through the Senate side of the Capitol for meetings to express his government's strong opposition, including a huddle with Majority Leader Mitch McConnell (R-Ky.).

Earlier, Yukiya Amano, the head of the U.N's International Atomic Energy Agency, met privately with members of the Senate Foreign Relations Committee to urge support for the proposal, and the day concluded with senators meeting in a classified briefing with Energy Secretary Ernest Moniz and senior advisers from the State and Treasury departments.

The momentum continued to build among Democrats for supporting Obama on this proposal. Sens. Chris Murphy (D-Conn.) and Angus King (I-Maine) voiced their support Wednesday, joining eight other Senate Democrats already backing the president. No Democrat in the Senate has announced opposition so far, although Sen. Robert Menendez (D-N.J.) has expressed deep misgivings about the plan.

A majority of Democrats on the Senate Foreign Relations Committee openly support the proposal and more are expected to announce their backing. One Republican, Sen. Jeff Flake (Ariz.), has signaled interest in the plan, tantalizing Senate observers with a speech Wednesday that tilted from support to opposition and back, before announcing that he had still not made up his mind.

"The bottom line is that I can only support an agreement that can endure," Flake said.