US Sen. Chuck Schumer (D-New York). AP US Sen. Chuck Schumer (D-New York) was planning to roll out a major decision to the American public on Friday.

Schumer, the Senate's No. 3-ranking Democrat who is next in line to lead his caucus, was set to reveal that he would buck the leader of his party — the president of the US — and vote against the Iran nuclear deal that has become a cornerstone of the president's emerging foreign-policy legacy.

Schumer went to the White House with the news Thursday, a courtesy heads-up. But hours later, The Huffington Post had the scoop, and a source close to Schumer said the leak sprung from the White House.

From top to bottom, liberals are amping up criticism against the man who's in line to become the next Democratic leader in the Senate.

Schumer angered the Democratic Party's left flank late Thursday night when he announced that he will vote to disapprove of the Iran nuclear deal when it comes to a vote in Congress.

A source close to Schumer also told Business Insider he would vote in favor of overriding a potential veto from President Barack Obama on the potential resolution of disapproval.

"I obviously profoundly disagree with the judgments made," Secretary of State John Kerry told reporters of the decisions of both Schumer and US Rep. Eliot Engel (D-New York) in opposing the deal.

The Schumer defection was by far the most significant disruption among a Democratic flank that had mostly united behind the president.

President Barack Obama with Chuck Schumer. AP

If Congress votes to disapprove of the nuclear deal, struck last month between Iran and six world powers, Obama has said he will veto the measure. A veto override would require at least 13 Democrats in the Senate and 44 in the House to break with their party and president — if all Republicans are united in their opposition.

The conventional wisdom holds that because of his stature and history of more hawkish foreign-policy views, Schumer could persuade other Democrats to follow his lead. The potential significance of his opposition was not lost on progressive organizations. The group MoveOn, for example, announced it would launch a campaign to halt donations to the Democratic Senatorial Campaign Committee and any Democratic candidate who opposes the Iran deal.

MoveOn's political-action executive director, Ilya Sheyman, also suggested Democrats should find a replacement for Schumer as the Senate's Democratic leader in 2017, when he is expected to take the place of retiring Senate Minority Leader Harry Reid (D-Nevada).

"No real Democratic leader does this. If this is what counts as 'leadership' among Democrats in the Senate, Senate Democrats should be prepared to find a new leader or few followers," Sheyman said. "This is not what the volunteers, activists, small-dollar donors, and voters who actually win elections spend their time and money to support."

Another progressive group, CREDO Action, branded Schumer as "Warmonger Chuck."

"Chuck Schumer was wrong on Iraq, and he is wrong on Iran," said Becky Bond, CREDO Action's political director. "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. Perhaps it is time to change his nickname from Wall Street Chuck to Warmonger Chuck."

The moves from those groups weren't exactly new. When Reid announced his retirement in March, MoveOn warned Schumer on his support for an Iran-related bill eventually signed into law by Obama. CREDO also attacked his history with Wall Street-related issues. President Barack Obama delivers remarks on a nuclear deal on August 5, 2015. REUTERS/Jonathan Ernst

As The Washington Post's Greg Sargent wrote Friday, the math remains in Obama's favor even with Schumer's defection. Three crucial Democratic senators came out in favor of the deal earlier this week. And hours before Schumer's decision was revealed, his state counterpart, Sen. Kirsten Gillibrand (D-New York), said she would support the deal.

But Schumer's opposition clearly irritated those closest to the president. Dan Pfeiffer, a former top aide to Obama, vented on Twitter late Thursday night, comparing Schumer's Iran-deal opposition to another issue on which he has long frustrated the White House and other Democrats: Obamacare.

Those still inside the White House took a more veiled shot at Schumer Thursday night — when the leak revealing his decision sprang. And on Friday, Eric Schultz, the White House's principal deputy press secretary, noted the support of Senate Democrats other than Schumer:

This post was updated at 12:42 p.m. with additional context.