Sen. Robert Menendez, a New Jersey Democrat, gave a speech Tuesday afternoon in New Jersey outlining his opposition to the deal and taking several shots at Obama in the process.
"I have looked into my own soul and my devotion to principle may once again lead me to an unpopular course, but if Iran is to acquire a nuclear bomb, it will not have my name on it," Menendez said. "It is for these reasons that I will vote to disapprove the agreement and, if called upon, would vote to override a veto."
The former ranking member of the Foreign Relations Committee has been an outspoken critic of the administration's negotiations with Iran and was widely expected to come out against the deal.
His comments directly took aim at Obama, who earlier this year tried to reassure critics that he had negotiated a good deal with Iran since his legacy would bear the consequences of it.
"This will be my name on it," the President said.
Obama's defense of the deal has rankled some in his own party, who feel he has attacked opponents of the agreement too harshly.
In an interview on CNN's "The Lead" after his speech, the New Jersey senator said the agreement was "aspirational" and that the U.S. was forfeiting whatever leverage it had left. When asked if maintaining the current sanctions regime was similarly "aspirational," Menendez said the status quo was sustainable.
"It will take work as it's always taken," Menendez said, stressing that the U.S. could retain the existing, pre-deal penalties. "The Congress has on many times rejected multilateral agreements, and the world hasn't come to an end,"
In a lengthy, point-by-point critique of the deal in his speech, Menendez pushed back on Obama's criticism of opponents, saying he was making his decision not based on politics but his own analysis.
"Unlike President Obama's characterization of those who have raised serious questions about the agreement, or who have opposed it, I did not vote for the war in Iraq, I opposed it, unlike the Vice President and the Secretary of State, who both supported it," Menendez said.
"I know that, in many respects, it would be far easier to support this deal, as it would have been to vote for the war in Iraq at the time. But I didn't choose the easier path then, and I'm not going to now," he added later, to applause.
Menendez referred to "countless hours" he has spent in hearings, classified briefings and serious discussions on the deal. "It is not an issue of supporting or opposing the President. This issue is much greater and graver than that," he said.
He even targeted Obama's signature message of "hope."
"Whether or not the supporters of the agreement admit it, this deal is based on 'hope' -- hope that when the nuclear sunset clause expires, Iran will have succumbed to the benefits of commerce and global integration," Menendez said. "Hope is part of human nature, but unfortunately it is not a national security strategy."
The New Jersey Democrat also countered Obama's assertion that rejecting the agreement would amount to choosing war.
"I reject that proposition, as have most witnesses, including past and present Administration members involved in the Iran nuclear issue, who have testified before the Senate Foreign Relations Committee," Menendez said. "For all those who have said they have not heard -- from anyone who opposes the agreement -- a better solution, they're wrong. I believe there is a pathway to a better deal."
Menendez said Congress should send the White House back into negotiations with specific requirements and guidelines.
"I believe we could still get a better deal and here's how: We can disapprove this agreement, without rejecting the entire agreement," he said.
Menendez's announcement follows that of Tennessee Republican Sen. Bob Corker, who criticized the deal in an op-ed
published Monday night.
"Many say now is the time for the United States to push back against Iran," Corker wrote. "The best way to do that is for Congress to reject an agreement that strengthens Iran with hundreds of billions of dollars over the next decade, removes the conventional weapons and ballistic missile technology embargoes on Iran and allows for a U.S.-approved, industrial-scale enrichment program for which Iran has zero practical need."
While Republicans have been expected to oppose the deal en mass, the White House had sought to engage a few key Republicans in hopes of persuading them to support the deal. Corker's position in the Senate makes him a key lawmaker when it comes to the deal. The White House also failed to win over Arizona Sen. Jeff Flake, who will stick with his party in opposing the deal.
Menendez will be only the second Senate Democrat to come out against the deal, after powerful New York Sen. Chuck Schumer announced his opposition earlier in the month.
On Tuesday, two more Democratic senators -- Rhode Island's Sheldon Whitehouse and Jack Reed -- jointly announced that they planned to support the deal.
Republicans will likely have the votes to pass a resolution of disapproval on the deal, but it's less clear if they could override a presidential veto. If all Republican senators were to vote against the president, they'd still need 13 Democrats to join them to reach the two-thirds threshhold in the upper chamber needed to overturn a veto -- and the House would have to do the same.
So far, Obama has gotten more supporters among his party than opponents, though not yet enough to guarantee victory.
Also on Monday, House Democratic Leader Nancy Pelosi's office circulated a letter from Reps. Adam Schiff of California, Jan Schakowsky of Illinois and Sander Levin of Michigan to other members of Congress that included a letter from 340 rabbis urging support of the Iran deal.
Israel and some in the American Jewish community have been among the biggest critics of the Iran deal because they say it would too substantially empower an enemy of Israel.
Despite the President being on vacation in Martha's Vineyard this week, the White House says Obama continues to engage on the Iran deal. The administration also predicted that a majority of Democrats will stand with the President on the deal.
"We are encouraged by the growing number of lawmakers who have announced support for the deal in the past week -- echoing the same arguments the President has been making for several months," deputy press secretary Eric Schultz said. "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."