WASHINGTON - As New Jersey Sen. Robert Menendez announced his firm (but unsurprising) opposition Tuesday to President Obama's nuclear deal with Iran, the votes of three other local Democrats hung in the balance, and with them critical tallies that could help determine whether the accord survives congressional objections.

Sens. Robert P. Casey of Pennsylvania, Cory Booker of New Jersey, and Chris Coons of Delaware all remain undecided on the pact as Obama, Secretary of State John Kerry, and other top officials try to persuade Democrats to stand with the president, and opponents, including Israeli leaders and some influential Jewish American groups, urge them to oppose what they describe as a dangerous agreement.

At stake is a potentially historic deal that Obama says is the most realistic option to prevent Tehran from obtaining a nuclear bomb, but that critics say is too soft and will pave the way to a nuclear Iran.

Republicans lining up in opposition will need only a handful of Senate Democratic votes to get to the 60 needed to pass a resolution disapproving the plan and deal an embarrassing blow to a pact that could cement Obama's international legacy.

Obama has promised to veto that resolution, though, so 67 Senate votes and two-thirds in the House would be needed to override him and kill the international agreement. That has put all eyes on wavering Democrats to see if they side with the president.

Unofficial counts have roughly 30 Senate Democrats either backing the agreement or leaning toward supporting it - including Sen. Tom Carper (D., Del.) - meaning that just a few more pledges of support would help Obama sustain a veto. Even more support could block the disapproval vote.

The only Republican senator from the Philadelphia region, Pennsylvania's Pat Toomey, opposes the deal.

Menendez on Tuesday became the second Senate Democrat to announce his rejection, joining Sen. Chuck Schumer (D., N.Y.), who is expected to lead Senate Democrats next year.

"If Iran is to acquire a nuclear bomb, it will not have my name on it," Menendez said in a speech Tuesday at Seton Hall University. He pledged to vote against the agreement and to override Obama's promised veto, if needed.

Republican opposition appears unanimous. But the prospects of overcoming a veto are fading as more Democrats back the agreement. Senate Majority Leader Mitch McConnell (R., Ky.) told reporters this week that Obama has "a great likelihood of success" given the high vote threshold.

The House will likely have enough votes to register its disapproval. At least one area Democrat, South Jersey Rep. Donald Norcross, came out in opposition to the deal Thursday night. It's unclear, though, if there is enough opposition in the House to overcome a veto.

Still, with votes looming in September, Casey, Booker, and Coons are facing pressure to join Menendez in bucking the president and helping pass a resolution that would signal disapproval of the deal.

Two pro-Israel lobbying groups - United Against a Nuclear Iran and Citizens for a Nuclear Free Iran - have blitzed the Philadelphia and New York airwaves with ads in recent weeks opposing the measure. (The Citizens group is allied with the influential American Israel Public Affairs Committee.)

A Jewish group that supports the agreement, J Street, has run ads on Philadelphia TV pushing in the other direction, and one liberal organization, Keystone Progress, has launched a petition urging Casey to side with the president.

All three undecided Democrats are usually loyal to the president, but have also forged ties to the Jewish community, parts of which have produced some of the most vocal objections, citing worries about Iran's threat to Israel. Like Schumer and Menendez, Casey, Booker, and Coons represent states with relatively large Jewish populations.

Aides to all three said the senators have been meeting with top administration officials and constituents, and poring over the details as they evaluate the agreement.

Booker plans to return to Washington on Thursday for further briefings; Casey plans to have an announcement the week beginning Aug. 31; and Coons will "likely" make his decision this month, aides said.

"I want to hear all angles on the deal," said a statement by Booker on Tuesday. "I am holding this deal to a very high standard."

Casey has promised a "thorough review," and has spoken with constituents on both sides of the issue, as well as top White House and cabinet officials and outside experts, a spokesman said.

A Coons aide said the senator, a member of the Foreign Relations Committee, has had conversations with Obama, Vice President Biden, Kerry, and other cabinet officials, as well as classified briefings.

"Most important, though, have been his direct conversations with Delawareans, including many who support the deal and many who don't," said the aide, Sean Coit.

Menendez's opposition was widely expected. He is a staunch ally of Israel and has fiercely criticized Obama's Iran policy.

The former top Democrat on the Foreign Relations Committee, Menendez said the final deal with Iran fell short of the goals set out at the beginning of talks and will leave Tehran close to developing a nuclear weapon while lifting punishing sanctions. Menendez stepped down from his post on the panel in the face of corruption charges this year.

"We want a deal, but we want the right deal," he said in a more than 6,000-word speech that delved into details about centrifuges, sanctions, and Iranian missiles. He urged Obama to seek tougher terms.

Obama's supporters say killing the agreement after long negotiations involving the United States, Britain, China, France, Germany, and Russia would fracture the international coalition that has imposed sanctions and sink U.S. credibility.

They see the deal as the best way to ensure that Iran does not obtain a nuclear weapon for the next 15 years. Without the pact, they say, Iran will remain on the brink of developing a nuclear bomb.

Obama has argued that the only other way to stop Iran from obtaining nuclear weapons will be another war in the Middle East - an idea critics strongly reject.

"We are encouraged by the growing number of lawmakers who have announced support for the deal," said White House spokesman Keith Maley, expressing confidence that a majority of Democrats will support the agreement and sustain a veto, if necessary.

Hours after Menendez's speech, Rhode Island's two Democratic senators endorsed the pact, bringing the president that much closer to overcoming opposition from Republicans and at least some Democrats.