(AP) -- Sen. Chuck Schumer, a New York Democrat expected to be the party's next leader in the Senate, said he will oppose the Iran nuclear deal in spite of President Barack Obama's intense lobbying in favor of the accord.

The deal, struck last month with Tehran and Western powers, would curb Iran's nuclear program in exchange for billions of dollars in relief from crippling sanctions.

"The very real risk that Iran will not moderate and will, instead, use the agreement to pursue its nefarious goals is too great," Schumer said in opposing the pact. He said he based his decision on the nuclear and non-nuclear elements of the accord and on the question, "Are we better off with the agreement or without it?"

A leading Jewish Democrat, Schumer was the first senator of Obama's party to step forward to oppose the deal. Obama had steadily been gaining support for the pact, and on Friday, Wisconsin Democratic Sen. Tammy Baldwin announced that she would back the measure.

"Simply put, I do not believe that rejecting this agreement is in our national security interest," Baldwin said.

Schumer's decision was a blow to the administration, though it remained to be seen how many other Democratic lawmakers would follow the New York senator. He informed the White House of his decision Thursday afternoon. New York Rep. Eliot Engel, who is Jewish and the top Democrat on the House Foreign Affairs Committee, said in a statement that he too would oppose the deal.

Secretary of State John Kerry said Friday he "profoundly disagrees" with the reasoning behind the lawmakers' decision to oppose the deal.

"It's a question of eliminating options in a realistic way," he said in Vietnam, where he was marking the 20th anniversary of U.S.-Vietnam relations. "I would respectfully suggest that rejection is not a policy for the future, it does not offer any alternative."

Kerry added that if the deal is rejected, "there will be a hue and cry about Iran's continued activity and that will lead people to put pressure on military action since the United States would have walked away from the diplomatic solution."

Schumer's split with Obama was remarkable for a senior leader in line to replace Senate Minority Leader Harry Reid, D-Nev., after he retires at the end of next year. His decision also put him at odds with the Democrats' likely presidential nominee, Hillary Rodham Clinton, who has cautiously embraced the deal.

The Senate's No. 2 Democrat, Dick Durbin of Illinois, supports the accord and has been working hard to persuade lawmakers to do the same.

The administration, which has lobbied intensely for the pact, has secured the backing of more than a dozen Senate Democrats and more than two-dozen House Democrats, including Minority Leader Nancy Pelosi, D-Calif. Republicans, who control the House and Senate, have been uniformly opposed to the deal.

Schumer signaled that he wouldn't lobby hard against the accord. The House and Senate will begin debate on a resolution of disapproval when lawmakers return to Washington on Sept. 8 after their August recess. The administration needs Democratic support to sustain a widely expected veto by Obama of any resolution of disapproval.

"There are some who believe that I can force my colleagues to vote my way," Schumer said. "While I will certainly share my view and try to persuade them that the vote to disapprove is the right one, in my experience with matters of conscience and great consequence like this, each member ultimately comes to their own conclusion."

Schumer had been under pressure as a congressional ally of Israel, leading fundraiser and strategist for his party, and lawmaker from a state that is home to more than a million-and-a-half Jews.

He complained that the pact does not allow inspections "anywhere, anytime" and that the United States cannot demand inspections unilaterally.

Schumer joined a handful of Jewish Democrats who have announced their opposition -- Reps. Steve Israel and Nita Lowey of New York and Ted Deutch of Florida. Michigan Rep. Sander Levin, however, has endorsed the deal.

The powerful pro-Israel lobby American Israel Public Affairs Committee is vehemently opposed to the deal, which Israeli Prime Minister Benjamin Netanyahu has denounced as undermining the security of Israel and the region. Opponents of the pact have targeted Schumer in campaign-style ads.

The media-friendly Schumer made the announcement through the blog Medium, not in a high-profile speech on the Senate floor like several of his colleagues. His statement also was posted as much of the political world was focused on the first Republican presidential candidate debate.

The announcement came hours after the Senate left Washington for its recess.

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AP Diplomatic Writer Matthew Lee contributed to this story from Hanoi, Vietnam.