WASHINGTON, July 14 (Reuters) - The nuclear deal between world powers and Iran starts a new phase of intense negotiation - this time between the Obama administration and the U.S. Congress, where some Republicans have long been working to sink an agreement.
Any effort in Congress to overturn the deal would face an uphill fight. Republicans have majorities in both the House of Representatives and Senate, but they would need the support of dozens of President Barack Obama's fellow Democrats to sustain a "resolution of disapproval" that could cripple a deal.
The chances of that happening are slim. A resolution of disapproval would need only the support of the Republican majority to pass the House, but would require the votes of at least six Democrats to get the 60 to advance in the Senate. The chances of mustering enough votes to then overrule a near-certain Obama veto are slimmer still.
The second-ranking Democrat in the House, Steny Hoyer, said after the deal was announced: "It is now up to members of Congress to work carefully through every detail, particularly given Iran's likelihood to exploit any ambiguity or loophole to its benefit and to the detriment of the security of America, Israel, and our allies in Europe and the Gulf."
Senate Democrats have stood firm to date against Republican-led efforts to interfere with the talks, which included Iran and the United States, Britain, China, France, Germany and Russia.
In the House, more than 150 Democrats, including party leader Nancy Pelosi, signed a letter in May strongly supporting the nuclear negotiations.
"I understand the heavy lift that's involved," Tennessee Republican Senator Bob Corker, chairman of the Senate Foreign Relations Committee, told reporters when asked about the chances of passing a "resolution of disapproval."
Obama in May signed a law, authored by Corker, giving Congress the right to review the agreement and potentially sink it by passing a disapproval resolution that would eliminate the president's ability to waive sanctions passed by Congress.
Easing sanctions is an integral part of the deal, under which Iran will curtail its nuclear program.
Under the Iran Review Act, lawmakers have 60 days to review the agreement and decide whether to seek a resolution of disapproval. During that period, plus an additional 22 days in which Obama could veto a resolution and Congress could try to override it, Obama cannot waive the congressional sanctions.
A veto override would require a two-thirds majority in both houses - or 13 Democrats along with all 54 Republicans in the Senate, and 43 Democrats plus as all 236 House Republicans.
Party leaders have said there is no guarantee that every Republican would back a disapproval resolution.
Sanctions passed by Congress account for the overwhelming majority of those imposed by the United States. U.S. sanctions are especially important to the international sanctions regime because of the country's influence on global trade and banking.
The congressionally mandated sanctions can be temporarily waived by the president for national security reasons, which he would do under the deal with Iran. That waiver ability stays in place unless Congress is able not just to pass a disapproval resolution, but override Obama's expected veto of it.
"Congress gave away its power by granting national security waivers with all of these sanctions," Corker said.
Congressional leaders said they plan to begin briefings and hearings on the Iran deal as soon as they receive the agreement, with an eye toward deciding on a course of action this month, before lawmakers leave for summer recess.
Obama administration officials, including the president himself, have reached out to members of Congress, holding hundreds of meetings and hearings and making telephone calls in the past four months. Those efforts are expected to intensify now with a deal.
Acknowledging the difficulty of passing a disapproval resolution, some lawmakers suggest that Congress would do better to consider, and then reject, a "resolution of approval."
Defeating such a resolution by a large margin would not affect the sanctions regime, but would send a strong message that the United States is not united behind a "bad" Iran pact and was prepared to act if Iran made moves toward building a bomb, they said.
Lawmakers from both parties acknowledged that the debate will not end with the review period this year. Some Republicans have discussed passing legislation to impose more sanctions over Iran's human rights record or for supporting terrorism.
The Iran Review Act requires the president, Obama and his successor after the 2016 election, to regularly certify that Tehran is adhering to terms of a deal. There is no guarantee a Republican president, in particular, would do so. Several of the GOP White House hopefuls have already said they are skeptical about the deal. (Additional reporting by Idrees Ali and Roberta Rampton; Editing by David Storey and Jeffrey Benkoe)