WASHINGTON -- New York Sen. Chuck Schumer, one of the most powerful Democrats in Congress, said Thursday night he would oppose President Barack Obama's nuclear deal with Iran.

Schumer, the highest-ranking Jewish Democrat in the Senate, has been a staunch supporter of Israel and was widely seen as skeptical of the negotiations with Iran over its nuclear capability.

The opposition of Schumer, expected to be his party's next leader, was announced several hours after New York's other U.S. Senator, Kirsten Gillibrand, also a Democrat, said that she would reluctantly support the deal.

PhotosHas Obama kept his State of the Union promises?Photos15 awesome moments between Obama and kidsMore storiesLatest news on President Barack Obama's second term

Schumer's split with Gillibrand reflects a larger divide among Democrats in Congress and creates a higher hurdle for the White House to win approval for its agreement.

In a statement Thursday night, Schumer said he would oppose the deal because "the very real risk that Iran will not moderate and will, instead, use the agreement to pursue its nefarious goals is too great."

Gillibrand said in a statement Thursday she supports the "imperfect" deal because without it "we do not have a viable alternative for preventing Iran from obtaining nuclear weapons."

advertisement | advertise on newsday

The positions of the two senators reflect the divergent views of how Iran might respond to, and the United States can enforce, the agreement between six nations led by the United States and Iran to curb its nuclear program in return for the lifting of economic sanctions.

They both raise issues that will be debated in the Senate when it returns from the August recess before Congress votes on the agreement by Sept. 17, as required by law.

Democrats are splitting on the agreement amid pressure from President Barack Obama and the White House ahead of a debate the Senate will hold when it returns from its August recess. Republicans are overwhelmingly opposed to the agreement.

@Newsday

Schumer and Gillibrand's announcements came during the week in which Long Island Reps. Steve Israel (D-Huntington) and Kathleen Rice (D-Garden City), as well as Rep. Nita Lowey (D-Westchester), said they will oppose the deal. Reps. Peter King (R-Seaford) and Lee Zeldin (R-Shirley) also have rejected it.

But like Gillibrand, seven other senators -- six Democrats and an independent -- this week said they support it.

Schumer and Gillibrand acknowledged weaknesses in the agreement, particularly the provision for a 24-hour delay for inspections when radioactivity is detected.

Gillibrand said Iran made essential concessions that include accepting inspections and monitoring of its nuclear supply chain, which puts the United States in a better position to act if Iran cheats.

"Our goal has been, and remains, to prevent Iran from developing a nuclear weapon," Gillibrand said. "We have far more ability to achieve that outcome if we approve this deal."

advertisement | advertise on newsday

But Schumer took a darker view of Iran's intentions and willingness to bide its time to build a nuclear weapon.

"Those who argue for the agreement say it is better to have an imperfect deal than to have nothing," he said. "When you consider only this portion of the deal -- nuclear restrictions for the first 10 years -- that line of thinking is plausible."

But Schumer added, "If Iran's true intent is to get a nuclear weapon, under this agreement, it must simply exercise patience. After 10 years, it can be very close to achieving that goal, and . . . Iran's nuclear program will be codified in an agreement signed by the United States and other nations."

Schumer and Gillibrand also disagreed on the likelihood that a better deal can be reached.

"Better to keep U.S. sanctions in place, strengthen them, enforce secondary sanctions on other nations, and pursue the hard-trodden path of diplomacy once more, difficult as it may be," Schumer said.

advertisement | advertise on newsday

But Gillibrand said she does not see that as a possibility.

"In a meeting earlier this week when I questioned the ambassadors of our P5+1 allies, it also became clear that if we reject this deal, going back to the negotiation table is not an option," Gillibrand said.

"Without a deal, our options will be limited to insufficient unilateral sanctions, an invasion with yet another massive and costly land war in the Middle East, or a bombing campaign that offers nothing more than short-term gain under the best-case scenario," she said.