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.
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.
The White House role
Then there's the outreach by the White House itself.
A senior administration official said Friday that the White House is "not flying by the seat of its pants" on its efforts to sell the deal. For months, long before the deal was even announced, an interagency team would huddle for several days a week to develop a playbook, dozens of pages long, on how to roll out the plan and tasking various administration officials on outreach in anticipation of opponents of the deal marshaling tens of millions of dollars.
"It is fair to say every heavyweight in the national security apparatus is involved on outreach to the Hill and other outside voices who matter, to give them the facts," the official said.
Engel and fellow New York Rep. Jerry Nadler have been among the Democratic congressmen to be recipients of that outreach, receiving individual meetings with President Barack Obama on the agreement.
While over his tenure in the White House, Democrats have complained that Obama has done very little personal outreach to his allies on Capitol Hill on various issues, this time he seems to be taking a different approach.
Following his meeting with Obama, Nadler said on Iran the President has been "certainly more hands on."
On Wednesday night, when a meeting the President was holding with the Democratic caucus was broken up by votes, he said that any member who wanted to ask questions was welcome to return. After the votes, a couple dozen spent nearly two hours with him.
White House Press Secretary Josh Earnest said Thursday that the President will continue to have many other conversations like these over the next several weeks, and seemed to acknowledge that the White House strategy is now focused on Democrats.
He said they weren't going to spend much time "trying to persuade" Republicans who came out against the deal before there was a deal. Instead, the deal's supporters are laser-focused on getting enough votes to sustain a veto.
Indeed, the GOP has its own plan for a campaign against the deal during the summer break, and most of its members have been clear that they will be voting against it.
Earnest disputed the notion that there is a "sense of urgency" right now due to concerns that they don't have enough Democratic support, saying the rush to meet with members now stemmed from so many of them leaving for the recess.
However, the President had a more dire message for progressive groups he spoke to by conference call on Thursday night.
"The lobbying that is taking place on the other side is fierce, it is well-financed, it is relentless," he warned groups including Organizing for Action, MoveOn and Americans United for Change.
"Right now the opponents of this deal have been flooding congressional offices, there are 20 million ads up right now in districts to put pressure on members of Congress. And they are feeling it," Obama told them.
He said their support was urgently needed since "I'm meeting with the members of Congress and they don't really buy the arguments of the opponents, but I can tell when they start getting squishy and they start getting squishy because they are feeling the political heat."
An acrimonious debate
The acrimony on the issue has spread from arguments over the deal to the nature of the debate itself.
The administration accuses opponent of waging a "misinformation" campaign that exaggerates the worst-case scenario of what a deal might portend.
Kerry, at a Senate hearing last week, slammed anti-deal TV ads arguing that a better deal could be had as a "unicorn arrangement involving Iran's complete capitulation."
The White House has also ramped up efforts to sell the agreement with a Web page -- full of fact sheets and info graphics -- and a Twitter account -- now with 18,000 followers -- promising to "set the record straight" about the deal. During the last few weeks of hearings about the deal, administration officials were tweeting directly to senators countering what it called distortion of the facts.
"The White House and rest of the administration will be aggressive across the board, from regional media, to Hispanic and African-American media, to the national press and any place else Americans are hearing about this deal." a senior administration official said.
Josh Block of The Israel Project, another organization opposing the deal, strongly rejected the administration's characterization of efforts to block it.
"The administration wants to talk in slogans and platitudes," he said. "They know the polls show the more the Americans learn about various aspects of this agreement, the more they don't want it. So it is incumbent on those of us who care about the deal to help people have a discussion and dialogue about what it would mean."
Gary Bauer, who heads the Christians United for Israel Action Fund, responded to the attacks by saying that the administration offers a "rosy" assessment of the deal, while his group's campaign is "based on facts about the serious dangers posed by the deal."
The recent Washington summit for the massive evangelical group -- which hopes to do for Israel what the National Rifle Association has done for the Second Amendment and gun rights -- featured appearances by six Republican presidential candidates and half a dozen members of Congress.
AIPAC may have more money, but CUFI has a much larger grassroots base. The group has 2.2 million members, including 1,500 pastors who can amplify the message during sermons.
When a final deal between Iran and world powers was announced in mid-July, thousands of CUFI members were deployed to lobby members of Congress. Bauer said an email action alert sent last week to its members resulted in upward of 100,000 emails to Congress to reject the deal.
"We are unique in that we have a large presence in every congressional district, in every county in America," Bauer said, stressing that CUFI wants to disabuse lawmakers of the misperception that a vote to support the deal will be soon forgotten.
"The temptation by politicians is to try and finesse these issues and move on, but no member will be able to claim ignorance," Bauer said. "CUFI will be there to remind them."