Senator Chuck Schumer
Senator Chuck Schumer Reuters

A "progressive" advocacy group in the US has pledged to withhold millions of dollars in campaign contributions to Sen. Chuck Schumer as well as other Democrats should they vote in Congress against the nuclear deal with Iran. 

Close to 18,000 people have signed a petition on MoveOn's website, called "No money for war hawks," the group told the Huffington Post on Friday. 

"After deep study, careful thought and considerable soul-searching, I have decided I must oppose the agreement and will vote yes on a motion of disapproval," Schumer explained Thursday to The New York Times

MoveOn, which has already launched a political campaign in favor of the Iran deal, refused to take Schumer's announcement in stride. 

"We thought Schumer and the Democratic Party had learned this lesson a decade ago after the war of choice in Iraq," Nick Berning, MoveOn’s communications director, told the Huffington Post.

"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.”

MoveOn, which touts itself as a "community of more than 8 million Americans...lead[ing] campaigns for progressive change," has committed in this latest petition to withhold $8.3 million that it would have otherwise donated to the Democratic Party.

It is affiliated with the American far-left, and has received backing by financier George Soros among others. 

MoveOn's controversial lobbying tactics have often come under fire, including for a member-submitted ad which compared President George W. Bush to Adolf Hitler during the 2004 elections. That video drew harsh criticism from the Anti-Defamation League, among others.

The group's latest move came less than 24 hours after Schumer announced his opposition to the nuclear deal signed between Iran and world powers last month.