By Matt Spetalnick, Patricia Zengerle and Idrees Ali
WASHINGTON (Reuters) - Top Republicans vowed on Wednesday to do their utmost to scrap President Barack Obama's nuclear deal with Iran as the biggest pro-Israel lobby geared up for an all-out campaign to pressure wary lawmakers into rejecting the agreement.
A bigger push against last week's historic accord in Vienna was being met with a counter-offensive by senior Obama administration officials, who have already spent hours on in-person and telephone briefings with members of Congress.
Secretary of State John Kerry, Energy Secretary Ernest Moniz and Treasury Secretary Jack Lew briefed the entire House of Representatives and Senate in separate closed-door sessions on Wednesday and will defend the deal at a public Senate Foreign Relations Committee hearing on Thursday.
As Congress opened a 60-day review of the deal, Republican U.S. House of Representatives Speaker John Boehner told reporters: "Because a bad deal threatens the security of the American people, we're going to do everything possible to stop it."
Obama insists that the Iran deal is the only alternative to more war in the Middle East.
The most influential pro-Israel group, the American Israel Public Affairs Committee (AIPAC), will deploy hundreds of lobbyists on Capitol Hill next Wednesday and Thursday to try to convince lawmakers, especially undecided Democrats, to vote against the deal, according to an official in the pro-Israel camp.
AIPAC is also conducting a national television advertising campaign sponsored by allied groups such as Citizens for a Nuclear Free Iran, U.S. pro-Israel sources said. They are expected to spend upwards of $20 million, one source said.
Under a bill reluctantly signed into law by Obama in May, Congress has until Sept. 17 to decide whether to approve or reject the agreement between Iran and world powers to rein in Iran’s nuclear program in return for sanctions relief.
Republicans control majorities in both houses of Congress. Many have come out strongly against the pact, which they say will empower Iran and threaten U.S. ally Israel.
Some said they wanted to know more.
Republican Representative Dennis Ross said he was predisposed against the agreement but, after the briefing, "I am probably inclined now to dig further and verify for myself."
But if Congress passes a resolution disapproving of the deal, dozens of Democrats would have to vote with them to override the Democratic president’s threatened veto, which is not likely in the fiercely partisan Congress.
"It’s a steep climb but not an impossible climb," the pro-Israel group official said of the coming campaign.
House Democratic Leader Nancy Pelosi has come out strongly in favor of it. Among the senior Democrats whom pro-Israel lobbyists hope to win over is Senator Chuck Schumer, a strong advocate for Israel's security who has yet to state his position.
Schumer told reporters as he left the hearing that he was still deciding. "It's a serious issue and I'm studying it carefully, giving it what it deserves," he said.
Several Republicans said the cabinet secretaries had not eased their concerns about several issues, particularly the ability to "snap back" sanctions if Iran violates the deal and the system for inspecting Iranian nuclear facilities.
Senator Ted Cruz, a 2016 Republican presidential candidate, said the agreement would provide Iran with billions of dollars that would be used to murder Americans and their allies.
"If this deal goes through, it will transform the Obama administration into the world's leading financier of radical Islamic terrorism," he said.
Deal opponents in the pro-Israel camp believe more lawmakers can be swayed by detailed arguments about what they see as loopholes that Iran could use to skirt the agreement.
Pressure from AIPAC, whose members' support is widely coveted, could also worry lawmakers up for re-election. AIPAC boasts 100,000 members.
At the same time, J Street, a smaller liberal pro-Israel group, is urging supporters to lobby Congress to support the Iran deal.
Kerry told reporters before the House meeting that the deal "will make the region, our friends and allies, safer. It will make the world safer … in the absence of any viable alternative."
(Additional reporting by Richard Cowan; Editing by Grant McCool and Lisa Shumaker)