Sen. Charles SchumerCharles (Chuck) Ellis SchumerAmerica isn't ready to let Sessions off his leash Schumer celebrates New York Giants firing head coach: ‘About time’ GOP should reject the left's pessimism and the deficit trigger MORE (D-N.Y.) is facing quick pushback after he said he will vote against the Iran nuclear deal, with one outside group saying he is "unfit" to be the next Democratic leader.

"Chuck SchumerCharles (Chuck) Ellis SchumerAmerica isn't ready to let Sessions off his leash Schumer celebrates New York Giants firing head coach: ‘About time’ GOP should reject the left's pessimism and the deficit trigger MORE was wrong on Iraq and he is wrong on Iran. Schumer's decision to join Republicans in attempting to sabotage the Iran nuclear deal once again shows that he is unfit to lead senate Democrats," said Becky Bond, the political director of CREDO Action.

Bond added that "perhaps it is time to change his nickname from Wall Street Chuck to Warmonger Chuck."

The strong remarks against Schumer come after the New York senator, who is expected to be the next Democratic leader, became the first Democratic senator to say he will oppose the Iran nuclear deal.

"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," he said on Thursday night. "To me, the very real risk that Iran will not moderate and will, instead, use the agreement to pursue its nefarious goals is too great."

Schumer's opposition undercuts what appeared to be growing momentum for the deal among Senate Democrats, with more than a handful coming out in support of the deal this week. 

Sen. Tammy BaldwinTammy Suzanne BaldwinDemocrats turn on Al Franken The Hill's 12:30 Report Avalanche of Democratic senators say Franken should resign MORE (D-Wis.) also announced her support on Friday morning after Schumer's announcement, saying that "it will best serve America's national security interests." 

But with roughly half of Senate Democrats still publicly undecided, the influential senator's decision to oppose the deal could sway some of his colleagues who are still on the fence.

CREDO isn't the only group targeting Schumer in the wake of his decision. MoveOn said that "Senate Democrats should be prepared to find a new leader or few followers." 

The group is asking its supporters to not donate to the Democratic Senatorial Campaign Committee or any Democrat running for reelection, including Schumer, who aren't support the Iran deal. 

Both progressive groups signed on to a letter earlier this year pledging to hold Democrats "accountable" who supported legislation that lets Congress review and vote on a deal. The letter was sent before the Obama administration withdrew its veto threat on the bill.