An American progressive advocacy group has committed to withholding millions of dollars in campaign contributions to Sen. Chuck Schumer (D-N.Y.) and other Democrats if their vote against the nuclear deal with Iran scuttles President Obama's diplomatic effort in September.

Nearly 18,000 people signed a petition on MoveOn's website, entitled "No money for war hawks," committing to withhold $8.3 million they would have otherwise contributed to the party less than 24 hours after Schumer announced he will oppose the deal in Congress, the group told the Huffington Post on Friday.

MoveOn, which describes itself on its website as a "community of more than 8 million Americans from all walks of life who use innovative technology to lead, participate in, and win campaigns for progressive change," has launched a campaign together with other supporters of the Iran deal aimed at urging constituents to pressure their elected officials to vote for the deal.

"We thought Schumer and the Democratic Party had learned this lesson a decade ago after the war of choice in Iraq. We want to demonstrate to those who haven’t made their decision yet that there will be substantial political consequences for those who want to take us to war,” Nick Berning, MoveOn’s communications director, told the Huffington Post.

According to a statement released by MoveOn, the group hopes to withhold $10 million in contributions within 72 hours of the launch of the "donor strike," The Hill reported.

Schumer, a senior Democrat, announced in a statement on Thursday that he opposes the Iran deal, saying that to him, "the very real risk that Iran will not moderate and will, instead, use the agreement to pursue its nefarious goals is too great."

At the same time, Rep. Eliot Engel, a member of the House Foreign Affairs Committee, also announced he will oppose the deal. "The answers I've received simply don't convince me that this deal will keep a nuclear weapon out of Iran's hands, and may in fact strengthen Iran's position as a destabilizing and destructive influence across the Middle East," he said.

Prime Minister Benjamin Netanyahu has been pushing lawmakers to oppose the nuclear agreement, which he considers a threat to Israel's survival. Some pro-Israel groups have also been spending millions of dollars on an advertising campaign to push members of Congress to vote no.

The U.S. Congress has until September 17 to vote on a resolution of disapproval of the Iran deal, which would eliminate Obama's ability to waive all sanctions on Iran imposed by the U.S. Congress, a key component of the agreement.

Obama has promised a veto if it is passed by the House and Senate.

Republicans would need dozens of Democrats to vote against Obama to override a veto so, while Thursday's announcements are a blow to the president, opponents of the deal still face an uphill battle to enact a resolution.

Several Democrats in both the House and Senate have already come out in favor of the nuclear deal. Schumer's colleague from New York, U.S. Senator Kirsten Gillibrand, announced her support on Thursday.