By ERICA WERNER and DAVID PORTER, Associated Press

WASHINGTON (AP) — The fiercely contested Iran nuclear deal will likely survive in Congress despite unified GOP opposition and some Democratic defections, the top Senate Republican says. That would mean a major foreign policy win for President Barack Obama.

Obama has "a great likelihood of success," Senate Majority Leader Mitch McConnell said in his home state of Kentucky this week — giving public voice to what other Republicans have acknowledged in private. "I hope we can defeat it, but the procedure is obviously stacked in the president's favor."

Indeed, even as Congress' August recess has hardened Republicans' opposition to the deal on Capitol Hill and on the presidential campaign trail, reality is setting in: They probably can't stop it. Significant Democratic defections from Obama would be required in both chambers of Congress, and even with opponents mounting a strenuous lobbying campaign in key congressional districts, such a prospect looks remote.

That means that even with Obama firmly in lame-duck territory and his GOP opponents in control of Congress and aiming for the White House, the president is on the verge of a legacy-defining victory on a pact that he and his supporters say will keep the world safe from Iran's nuclear ambitions. Opponents continue to warn furiously that the result could be just the opposite: to strengthen Tehran's hand, in an existential threat to Israel and the world.

On Tuesday a second Democratic senator, Bob Menendez of New Jersey, did announce his opposition to the deal, joining Chuck Schumer of New York.

"The agreement that has been reached failed to achieve the one thing it set out to achieve — it failed to stop Iran from becoming a nuclear weapons state at a time of its choosing," Menendez said in a blistering speech at the Seton Hall School of Diplomacy and International Relations in East Orange, New Jersey. "In fact, it authorizes and supports the very road map Iran will need to arrive at its target."

Menendez argued the deal should be sent back and negotiations should continue. But his opposition was expected, and, underscoring slim prospects for his side, he stopped short of predicting opponents would prevail.

The agreement would require Iran to dismantle most of its nuclear program for at least a decade in exchange for billions of dollars in relief from international sanctions. But the Israeli government and critics in the U.S. argue that it would not stop Iran from building a bomb.

Bipartisan legislation does give Congress the right to review the deal, and there will be a vote by Sept. 17. That's likely to go in favor of disapproval, but Obama would then veto the legislation and opponents would need to muster two-thirds majorities in both chambers to override him.

Obama needs support from 34 of the 46 members of the Democratic caucus to sustain a veto, and 23 have already announced they are backing him. Sens. Jack Reed and Sheldon Whitehouse of Rhode Island issued a joint release in support of the president Tuesday just hours after Menendez declared his opposition. In the House, 146 of the 188 Democrats are necessary to sustain a veto, and more than 50 have expressed their support for the accord, compared to 10 opponents.

From his vacation home on Martha's Vineyard, Obama has been lobbying the undecided. The White House said Obama has talked individually or in small groups with nearly 100 lawmakers since the deal was announced last month, with Cabinet and senior administration officials reaching out to dozens more.

"We remain confident that ultimately a majority of Democrats in both the House and the Senate will support the deal, and if necessary, sustain the president's veto," White House spokesman Eric Schultz said Tuesday.

Liberal and progressive groups are joining in the lobbying, while opponents on the right including the American Israel Public Affairs Committee are spending millions to try to build opposition.

Any chance of the White House winning GOP support for the deal in Congress evaporated in recent days as Sen. Jeff Flake of Arizona, who had been seen as a possible "yes" vote, declared he would vote "no." Sen. Bob Corker of Tennessee, the chairman of the Foreign Relations Committee, announced his opposition in an opinion piece Tuesday in the Washington Post, and House Speaker John Boehner of Ohio has been criticizing the deal as he travels the country by bus raising money for Republican lawmakers.

But the deal has picked up scattered support from Republicans outside of Congress, including former Sen. Richard Lugar of Indiana, who chaired the Foreign Relations Committee, and former Treasury Secretary Hank Paulson. Predictions that the issue would dominate Congress' August recess have yet to come true, and Obama still finds himself in a strong position to prevail.

"It's not easy to override a president's veto when the president is so committed to getting this done, and really the White House is fighting very hard for every vote in Congress," former Democrat-turned-independent Sen. Joe Lieberman, who is helping marshal opposition, said in an interview Tuesday. But Lieberman said there were still enough undecided votes to get to two-thirds, and "we're working hard on them."

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Porter reported from East Orange, New Jersey. Associated Press writer Darlene Superville contributed from Edgartown, Massachusetts.