Sen. Charles Schumer became the most senior Democrat to break ranks with the administration on the deal. (Susan Walsh/AP)

Sen. Charles E. Schumer announced his opposition Thursday night to President Obama’s nuclear pact with Iran, becoming the most senior Democrat to break ranks with the administration.

The likely next leader of Senate Democrats, Schumer (N.Y.) said his three-week reading of the proposal left him unconvinced. “There is a strong case that we are better off without an agreement than with one,” he said, referring to the nuclear pact.

He made his announcement in a statement posted on Medium, the online forum, and at a time when much of the political world was focused on the GOP presidential primary debate in Cleveland.

Schumer is in line to succeed Minority Leader Harry M. Reid (D-Nev.), who is retiring at the end of 2016. He is also the most senior Jewish member of Congress.

The Senate left Washington on Wednesday for a five-week break. Majority Leader Mitch McConnell (R-Ky.) has said a debate will begin on the issue when it returns.

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Under the deal, Obama will be able to lift sanctions that he and previous presidents imposed on Iran. But the remaining sanctions, imposed by Congress, will have to wait until the review process is finished on Capitol Hill.

Opponents of the deal with Iran must ensure that they reach a two-thirds majority in each chamber — a long shot — to overcome an Obama veto of a so-calledresolution of disapproval.

While his opposition is significant, Schumer did not signal whether he would actively rally undecided Democrats to also oppose the deal.

“Supporters argue that after ten years, a future President would be in no weaker a position than we are today to prevent Iran from racing to the bomb,” he wrote. “That argument discounts the current sanctions regime. After fifteen years of relief from sanctions, Iran would be stronger financially and better able to advance a robust nuclear program.”

Schumer’s decision brought condemnation from some Obama allies who are still upset with his proclamations that the timing for pushing the president’s health-care law through Congress in 2010 was wrong politically. 

“The base won’t support a leader who thought Obamacare was a mistake and wants War with Iran,” Dan Pfeiffer, the former Obama adviser, wrote on Twitter late Thursday, embracing the president’s line that the only way to otherwise curtail Iran’s nuclear ambitions was war. 

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