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Story highlights

Anti-deal advocacy groups flush with cash are flooding congressional offices with calls, emails to stop the Iran nuclear deal

The White House has launched an all-hands-on-deck approach involving nearly every major national security official to defend the deal

(CNN) —  

The sides facing off over the Iran nuclear deal are pulling out all the stops in an epic Capitol Hill battle.

Anti-deal advocacy groups flush with cash are flooding congressional offices with calls and emails, running ads in national media and employing other campaign-style tactics to sway skeptical lawmakers before a mid-September vote on the agreement. Meanwhile, the GOP is readying its own campaign to take place over the summer recess.

For its part, the White House has launched an all-hands-on-deck approach involving nearly every major national security official. Secretary of State John Kerry and Defense Secretary Ashton Carter are leading public and private classified briefings. Vice President Joe Biden is hosting a series of congressional meetings and intimate get-togethers while President Barack Obama is even holding one-on-one sessions with some undecided Democrats. Obama and Biden have also held conference calls with interest groups and concerned activists.

Both camps also have a diplomatic corps on their side. Ambassador Ron Dermer of Israel, which has leveled intense criticism at the deal, is visiting dozens of congressional offices. On the other side, representatives from Britain, France and Germany, which joined with the U.S. to negotiate the deal, have been fervently knocking on members’ doors and hosting them in their lavish diplomatic residences.

Even veterans of Washington’s many political battles say the effort stands out for its breadth, funding – and stakes.

The administration argues that the deal it hammered out over 18 months of tense negotiations is the best, and at this point only, way to keep Iran from getting a nuclear weapon.

Opponents claim that the terms of the agreement are so lax it paves the way for a country that has called for Israel’s destruction, threatens its Sunni neighbors and is a leading state-sponsor of terror to get the ultimate tool of military destruction.

A historic mobilization

“From our perspective, this is one of the most significant mobilization efforts in our organization’s history,” said an official from the American Israel Public Affairs Committee, or AIPAC, the lobbying group leading the anti-deal charge. “It is certainly the most important issue in a generation and we are fully engaged.”

AIPAC, like other opponents of the deal, is calling for lawmakers to vote against the deal when a 60-day review period ends on September 17. The Republican-controlled Congress likely has the votes to pass a resolution of disapproval, which would block the deal. But Obama has promised to veto it, and Republicans would need to attract support from several Democrats to override it.

AIPAC officials say they expect their 100,000 members to meet with every member of Congress before the vote, in home districts and recess town halls as well as in Washington.

On Wednesday several hundred AIPAC members met in Washington with more than 400 members of Congress from both sides of the aisle and on both sides of the issue.

AIPAC has also formed a tax-exempt lobbying group to rally opposition to the deal. Citizens for a Nuclear Free Iran has recruited several prominent Democratic operatives, including pollster Mark Mellman and media consultant Mark Putnam, and former Democratic lawmakers including Sens. Evan Bayh, Mary Landrieu and Joseph Lieberman to sit on its advisory committee.

The group is starting to spend upward of $20 million on a 35-state ad campaign, including New York, where prominent Democrats like Sen. Charles Schumer and Rep. Eliot Engel, who are Jewish themselves and have large Jewish constituencies skeptical of the deal, remain undecided amid intense pressure from the White House.

READ: Schumer still mum, but White House gets key Iran deal backer

Schumer is one of the 15 Senate Democrats who remain on the fence. Schumer, who is close to AIPAC, has promised to review the agreement with a “fine-tooth comb.”

On Monday, he stressed that he wouldn’t let “pressure, party or politics influence” his vote. And he suggested that he wouldn’t be making his decision any time soon, noting that he has read the agreement several times and is still “studying it extremely carefully.”

Schumer is poised to become the Senate Democratic leader in the next Congress and is very influential with rank-and-file Democrats, so his arguments will be watched very closely by others on the fence.

One pro-Israel lobbyist called convincing Schumer the “lynchpin” of lobbying efforts. AIPAC and other lobbying groups, including Christians United for Israel, or CUFI, are flooding the offices of Schumer and other New York Democrats with emails and calls, reminding them about the dangers of the deal.

There are also groups pushing back on the other side, chief among them J Street. The lobby has just a fraction of the tens of millions of dollars AIPAC has dedicated toward the effort, and AIPAC’s ties on Capitol Hill are much wider and deeper, but the liberal group says its views are more representative of those held by Jewish Americans, a majority of whom backed the deal in a recent poll by the Los Angeles-based Jewish Journal.

J Street is just one tool in the Obama administration’s arsenal to win support for the deal. Additional advocacy groups, foreign policy experts, former U.S. officials and European diplomats have also been enlisted to counter what one senior administration official called, “the massive efforts by AIPAC, CUFI and other pro-Israel groups.”

British Ambassador Sir Peter Westmacott told CNN that speaking to members of Congress “has taken up quite a big part of my time recently.”

He points out that the deal is not just between the U.S. and Iran but was negotiated and agreed to by the UK, France, Germany, China and Russia, America’s negotiating partners over the past several years, and that he is sending the message that the international community is united on the deal being a “good and verifable” one.