Under the agreement, which the U.S. and other world powers negotiated with Tehran, Iran would curb its nuclear program in exchange for billions of dollars in relief from economic sanctions, which have been choking its economy.
Menendez, a senior member of the Senate Foreign Relations Committee, joins Sen. Chuck Schumer of New York in rejecting the deal.
Menendez said his opposition is not an issue of whether he supports or opposes Obama, who has pledged to veto a congressional resolution of disapproval. He said he is opposed because Iran has violated various U.N. Security Council resolutions while advancing its nuclear program and that the agreement doesn't require Iran to dismantle its nuclear infrastructure.
"Let's remind ourselves of the stated purpose of our negotiations with Iran: Simply put, it was to dismantle all — or significant parts — of Iran's illicit nuclear infrastructure to ensure that it would not have nuclear weapons capability at any time. Not shrink its infrastructure," Menendez said.
Senate Majority Leader Mitch McConnell has expressed doubts that Congress could override Obama's expected veto. Twenty-one Senate Democrats and Independents of the 34 needed to sustain a veto are backing the deal. Schumer, the No. 3 Democrat in the Senate and the party leader-in-waiting, is the only other notable Democratic defection.
In the House, at least 50 Democrats have expressed support. Democratic leader Nancy Pelosi of California has spoken confidently about rounding up the votes to save the deal. Ten House Democrats have announced their opposition.
Menendez urged the Obama administration to authorize the continuation of negotiations and recommended several changes, including requiring Iran to allow permanent access to suspect sites; a ban on centrifuge research and development for the duration of the agreement; an extension of the agreement to at least 20 years, and authorizing Israel to "address the Iranian threat on their own" if Iran accelerates its nuclear program.
"We must send a message to Iran that neither their regional behavior nor nuclear ambitions are permissible," he said. "If we push back regionally, they will be less likely to test the limits of our tolerance towards any violation of a nuclear agreement."