When Senator Chuck Schumer went public last week with his opposition against the nuclear deal negotiated by the Obama administration and five other major powers with Iran, the White House immediately switched to damage control mode.

Press Secretary Josh Earnest quickly tried to downplay the New York Senator's decision saying it was "not particularly surprising to anybody here at the White House." But Earnest's suggestion that fellow Democrats should consider Schumer's stance when the influential Democrat is up to become his party's next leader in the Senate in 2017 shows that Schumer's public break stung the Obama administration.

Schumer was not alone. Representative Eliot Engel, another lawmaker from New York, declared his opposition to the deal on the same day.

Blow for Obama

"There is no question this is a serious setback for the administration's efforts," said Dalia Dassa Kaye, director of the Center for Middle East Public Policy at the Rand Corporation. "Both these members of Congress are viewed as strong supporters of Israel, so their opposition makes it more challenging for the administration to argue this deal is not bad for Israel. That said, other leading Democrats are in favor of the deal, including prominent Senators like Diane Feinstein, who also have strong pro-Israel records."

For Republicans, who have vowed to vote against the deal, but also for Israel's Prime Minister Benjamin Netanjahu who even went before Congress to lobby against it, Schumer's decision raises the hopes that other still undecided Democrats may follow suit.

"I think it will provide cover for some Democrats - like Engel - who where already inclined to oppose the deal," said Miles Pomper, a Senior Research Associate with the Center for Nonproliferation Studies. "But each lawmaker will be juggling a number of different influences."

Two-thirds majority needed

Republicans have already promised to pass - and they probably do have the votes - a resolution to nix the accord after Congress comes back from recess in September. President Obama meanwhile has already promised to veto any attempts to kill the deal.

To override his veto, Republicans, would need a two-third majority in both chambers of Congress; a tall order for the GOP, because it would need to get a large number of undecided Democrats to switch and vote with them to sink the Iran deal.

The Obama administration is now expected to increase its efforts to secure enough votes in Congress to try to ensure the eventual passage of the agreement.

"President Obama is likely to sway Democrats on two fronts - the substance of the deal and the political ramifications of rejecting the deal," said Kaye. On the substance, she added, Obama will continue to press his argument that blocking the deal, "leaves war as the only other option, and few Democrats are interested in another costly war in the Middle East."

Republican Senate Majority Leader Mitch McConnell already blasted Obama's pitch for the Iran nuclear deal, saying it was "absurd" to argue that lawmakers must essentially choose between the agreement or going to war. "It's not this deal versus war", he said. "That's the argument they've been making during the whole negotiation. It's either this deal or a better deal, or more sanctions."

Obama's legacy

But Obama and his allies will also drive home another message to fellow Democrats, namely that killing the agreement would be a major blow not only to Obama, but also hurt the party in upcoming elections.

And the White House certainly has the tools to make lawmakers think twice before voting to kill what the administration considers a signature foreign policy success.

"He has considerable leverage, both in favors and side-deals he can cut or threats he can make and attention he can provide to lawmakers, the needs for which probably went up after the Schumer announcement," said Pomper.

Asked for their predictions, both experts think the Iran deal will stand in Congress.

"Yes, although it's likely to be very close," said Kaye.

"Yes, in the end most Democrats realize that if Obama loses this vote it will gravely harm his presidency for the remainder of his term," said Pomper. "It is not in their interest to see him fail."