The American Israel Public Affairs Committee (AIPAC) said in a statement it was “deeply concerned” that the deal “would fail to block Iran’s path to a nuclear weapon and further entrench and empower the leading state sponsor of terror.”

The considerable clout of pro-Israel interests on Capitol Hill will play an important role in deciding the fate of the pact, hammered out in Vienna after many months by Iran, the United States, Britain, China, France, Russia and Germany.

Congressional votes on the deal, which got a rough initial reception from Republican lawmakers, were not expected until September. Regardless of Israeli lobbying, however, odds were slim that U.S. lawmakers would be able to derail the deal.

AIPAC has 11 registered lobbyists in Washington and spends about $3 million a year on lobbying, according to the non-partisan Center for Responsive Politics, a watchdog group that monitors lobbying expenditures and campaign finance.

AIPAC is widely viewed as the most influential group in the United States advancing the Israeli government’s agenda.

“Few lobbies dedicated to international issues are so active and well-financed as the Israel lobby,” the center said of pro-Israel organizations generally.

Israeli Minister of Public Security Gilad Erdan, speaking to Israel’s Army Radio, said his government “must focus and explain all of the holes in this agreement” and “hopefully the Congress and Senate will see the truth.”

The agreement with Iran is seen as a legacy project for Democratic President Barack Obama, potentially more sweeping than his bold move to normalize U.S. relations with Cuba after decades of strife.

Not since 1981, when then-President Ronald Reagan proposed selling advanced surveillance aircraft known as “AWACS” to Saudi Arabia, has a U.S. administration been so embroiled in a fight with the Israeli government, normally a close U.S. ally.

Bruce Riedel, a former CIA official who is now a senior fellow at the Brookings Institution, told Reuters that in the fight over the Saudi AWACS, which Israel lost, it won a “green light” from the United States for its invasion of Lebanon just months later.

Leslie Gelb, a former State Department official responsible for arms sales in the 1970s, said in an interview that when the AWACS-Saudi sale began emerging during Jimmy Carter’s presidency, Israel began clamoring for American cluster bombs, advanced helicopters and F16 jets as a counterbalance.

Another weapons shopping list could be in the mix again, according to experts.

Some of the lobbying will be aimed at Jewish members of Congress, who will be influential voices in the upcoming debate. Two of them are so far withholding judgment: Senator Benjamin Cardin, the senior Senate Foreign Relations Committee Democrat; and Senator Charles Schumer, the No. 3 Senate Democrat.

Besides lobbying, 2016 political campaign contributions to members of the U.S. Congress are expected to be dangled.

In 2014, pro-Israel groups contributed $11.9 million to congressional candidates, with $6.8 million going to Democrats and $5.1 million to Republicans, according to the center.

Among the top recipients were Senate Majority Leader Mitch McConnell, Senator John Cornyn, the No. 2 Senate Republican, and Representative Steny Hoyer, the No. 2 House Democrat.

Meanwhile, Sheldon Adelson, a U.S. billionaire businessman and outspoken critic of the Iran negotiations, could also use his vast financial resources to try to influence Congress.

In 2012, Adelson pumped $92.8 million into Republican “super PACs,” the center said, making him the single highest contributor to outside groups that year.

Some pro-Israel groups will be arguing for Obama’s pact. Americans for Peace Now has welcomed the agreement as one that will “verifiably roll back Iran’s nuclear program.”

Ori Nir, a spokesman for the group that advocates an Israeli-Palestinian and Israeli-Arab peace, said his organization is calling on supporters to “start contacting members of Congress to influence them to support the deal.”—Reuters