Sen. Robert Menendez, a prominent Democrat on the Senate Foreign Relations Committee, announced Tuesday that he will vote against the Obama administration’s nuclear agreement with Iran.

Menendez, of New Jersey, is the second prominent Senate Democrat to publicly oppose the deal before next month’s vote, following Sen. Charles E. Schumer of New York.

His announcement, made in a speech at Seton Hall University, is a potential setback for the administration as it seeks the support of enough Democrats to prevent Congress from overriding Obama’s planned veto of any resolution that would sink the agreement. But it was not a surprise. Menendez has been very critical of the deal finalized in Vienna last month, and he was seen as unlikely to be won over by the White House.

“If Iran is to acquire a nuclear bomb, it will not have my name on it,” Menendez said in an advance copy of his speech . Menendez said he not only will vote against the Iran agreement but also would vote to override a veto.

So far, 23 of the 34 senators needed in the Senate to block an override of an Obama veto have announced their support for the deal, in which Iran accepts restrictions on its nuclear program in exchange for the lifting of international sanctions. The number was boosted by two Tuesday, when two more Democratic senators, Jack Reed and Sheldon Whitehouse, both of Rhode Island, announced their support for the deal.

Where lawmakers stand on the Iran deal

Administration officials have said they are optimistic about their chances. But Obama has continued to lobby lawmakers by phone during his two-week vacation in Martha’s Vineyard. White House allies, including some Jewish American groups, have joined the effort to win Democratic support.

On Tuesday morning, more than 70 arms control and nonproliferation experts endorsed the deal in a letter released by the Arms Control Association, a nonpartisan group based in Washington. And on Monday, a letter signed by 340 rabbis was released in support of the deal.

But opponents are mounting a fierce counterassault, including other groups such as the American Israel Public Affairs Committee, which has launched a multimillion-dollar advertising campaign against the deal.

Last Saturday, Sen. Jeff Flake (R-Ariz.), who had been heavily lobbied by the White House, announced he would oppose the deal, and Sen. Bob Corker (R-Tenn.), chairman of the Foreign Relations Committee, said Monday he also would oppose it, leaving the administration unlikely to attract bipartisan support.

Menendez in his speech accused negotiators from the United States and its five negotiating partners of squandering leverage created by sanctions that have crippled the Iranian economy and said they should have walked away from the talks.

“It is difficult to believe that the world’s greatest powers — the U.S., Great Britain, France, Russia, China, Germany and the European Union, sitting on one side of the table, and Iran sitting alone on the other side, staggering from sanctions and rocked by plummeting oil prices — could not have achieved some level of critical dismantlement,” he said.

Menendez said the negotiations began on the premise that its nuclear infrastructure would be largely stripped away. The agreement calls for Iran to reduce its stockpiles of fissile material, put most of its uranium-enriching centrifuges into storage and allow international inspectors to monitor compliance. But it allows Iran to continue a nuclear program for nonmilitary purposes such as energy production and medical isotopes.

Sen. Robert Menendez (D-N.J.) said he will vote against the Iran nuclear deal next month. (AP)

“We have now abandoned our long-held policy of preventing nuclear proliferation and are now embarked, not on preventing nuclear proliferation, but on managing or containing it,” Menendez said.

“The agreement that has been reached failed to achieve the one thing it set out to achieve,” he added. “It failed to stop Iran from becoming a nuclear weapons state at a time of its choosing.”

Menendez dismissed many of the administration’s arguments against a congressional rejection of the deal, saying he doubted that most countries of the world would risk business ties with the United States in favor of pursuing opportunities in a much smaller Iranian economy.

He also proposed that the administration go back to renegotiate what he called a “better deal.” Ambassadors of the five countries that negotiated the deal alongside the United States, however, have told lawmakers that the deal is not open for renegotiation.

But Menendez said Iran might go along if it continues to receive part of its frozen assets, sweetened by a one-time release of funds as a “good-faith down payment” on the talks.

It is unclear how much influence Menendez’s announcement will have on his colleagues. He no longer is the ranking Democrat on the committee, so his opposition is less meaningful than it once was. But his words may sway some of the still-undecided senators in the Mid-Atlantic region.

Karoun Demirjian, Paul Kane and David Nakamura contributed to this report.