As U.S. and Iranian negotiators approach the June 30 deadline to reach a nuclear deal, America's largest pro-Israel lobby is campaigning to kill such an accord in Congress.

Since last month, the American Israel Public Affairs Committee has mobilized its members to press legislators to endorse five principles for a nuclear deal -- principles that are almost certain not to be reflected in a final agreement.

Parallel to this campaign, major donors to AIPAC and other pro-Israel causes are forming a new and independent 501(c)(4) advocacy organization, according to fundraisers and other lobbyists involved in the effort. The new organization will buy TV, radio and Internet ads targeting lawmakers from both parties who are on the fence about the nuclear deal, these sources say.

Officially, AIPAC is still reserving judgment on the nuclear deal being ironed out now in Vienna by the U.S., Iran and five other world powers. But it's clear that the agreement now being negotiated would be unsatisfactory to AIPAC. For example, AIPAC's principles say a deal should last "decades," while the framework for the nuclear agreement released in April would begin easing restrictions on Iran's program after a decade. Another principle says inspectors must be given "anytime, anywhere" access to suspected sites, "including all military facilities." Iran's leaders have consistently said there will be no inspections on military sites.

AIPAC is now prepared to fight the White House by pushing Democrats to vote against the president's signature second-term foreign policy initiative.

That represents a break from how AIPAC usually does business. While the lobby has pressed Congress to sanction Iran since the early 1990s, it rarely opposes a sitting president on major votes. Despite initial misgivings, the Obama White House ended up supporting major sanctions against Iran in 2010, 2011 and 2012, avoiding a fight with AIPAC. In early 2014, AIPAC backed away from its plan to push for a sanction vote during the nuclear negotiations, in the face of White House pressure on Democrats to give negotiators room to strike a deal. AIPAC supported legislation this year to give Congress a chance to review and disapprove the Iran deal; the White House initially opposed this measure. But the administration dropped its opposition after some changes were made to the bill that made it procedurally more difficult to disapprove the deal.

The campaign against the Iran deal is focused on that vote. Depending on when the White House submits the nuclear agreement to the House and Senate, Congress will have 30 or 60 days to review it, which is sure to have complex twists and turns. Then lawmakers can vote to approve or disapprove the deal. If Congress can sustain a veto-proof two-thirds majority in the House and the Senate, then Obama will not be allowed to waive the sanctions on Iran legislated by Congress. (He will still be allowed to waive sanctions imposed through executive authority.)

Getting two-thirds of the House and the Senate is no small matter. As I reported in April, House Speaker John Boehner acknowledged to a private meeting of the Republican Jewish Coalition that Republicans didn't have the votes to overcome an Obama veto on a resolution disapproving the Iran deal.

But this doesn't mean that AIPAC and other pro-Israel groups will not at least make a fight out of it. Last week AIPAC's national council was in Washington for a previously scheduled meeting. But the group's national leaders also took the opportunity to make the case for the five principles to members of Congress, according to Congressional staff who spoke on the condition of anonymity.

For those activists who couldn't make it to Washington, AIPAC members are also reaching out through the mail. One AIPAC mailing I obtained being circulated among Florida pro-Israel activists was a letter to Debbie Wasserman Schultz, the Florida Democrat and chairwoman of the Democratic National Committee. "There is still time to influence the agreement," the letter says, "and as a member of Congress who represents one of the largest pro-Israel constituencies in the country we urge you to play a role in ensuring that Congress does not accept a deal that does not eliminate the Iranian pathway to a nuclear weapon."

Josh Block, the president and chief executive officer of the Israel Project and a former spokesman for AIPAC, told me that the campaign on the Iran deal is helped by the accord itself. "The more people learn about the developing deal, the more alarmed and opposed they are to these terms." To that end, Block said his organization will be traveling to Vienna for the negotiations and will be revamping a digital social media campaign advocating against what it considers to be a bad deal through a website it created about Iran's nuclear program.

The new 501(c)(4) group funded by AIPAC donors, according to one pro-Israel lobbyist familiar with the campaign, will focus on about a dozen media markets with large Jewish populations to make the case against the Iran deal, targeting Republicans and Democrats. Other similar 501(c)(4) groups have already started these kinds of ad buys.

One such group, Secure America Now, has produced video ads featuring Maria, a woman whose father was killed in Iraq by an improvised explosive device supplied by Iran. In the ad, Maria tells the camera, "And now President Obama would do a deal that lets Iran get a nuclear weapon."

The Emergency Committee for Israel, another 501(c)(4), is also considering running ads against the Iran deal in several targeted media markets, according to its executive director, Noah Pollak.

Other advocacy groups are also planning to run ads in sensitive political districts. Chris Maloney, a spokesman for the American Security Initiative, whose board includes four former senators -- Norm Coleman, Saxby Chambliss, Joe Lieberman and Evan Bayh -- told me his group is about to launch an ad buy tallying just under $1.4 million, to target eight Republican and Democratic senators. The ad buy will include television and digital media.

Maloney told me that one target of that campaign will be Senator Chuck Schumer, a New York Democrat who is in line to be the leader of his party in the Senate after Harry Reid retires. Schumer has not yet said whether he will support the Iran nuclear deal. But he has in recent speeches to Jewish groups outlined his own criteria for evaluating it.

Schumer's criteria are not exactly the same as AIPAC's, but he shares some of their concerns. According to a recording provided to me of Schumer's remarks on June 18 before an AIPAC event in New York, Schumer said that an outright ban on inspections of military sites would be unacceptable. "Inspections have to be across the board everywhere and they have to be quick and they have to be unilateral on behalf of the United States," he said.

In the past Schumer has given the president deference to negotiate an Iran deal while other pro-Israel groups have pressed for more sanctions. So far he has not paid a political price among this constituency. But they are watching his vote and those of his colleagues very carefully. 

One of those people watching is William Kristol, the editor of the Weekly Standard and the chairman of the Emergency Committee for Israel. "Whether a deal is approved or disapproved, the fight to prevent a nuclear Iran will continue, as will the need to hold members of Congress accountable for their votes, and presidential candidates accountable for their positions," he told me. "A vote for a bad Iran deal is a vote that will live in infamy."