CNN  — 

Twenty-two leaders of national progressive organizations signed an open letter calling for a “massive, nonviolent mobilization to stand up against (Donald) Trump’s bigotry and incitement of violence” and pledging to fight his presidential campaign.

In the letter released on Tuesday, signatories – which include leaders of, the Service Employees International Union, the Sierra Club, Greenpeace, the National LGBTQ Task Force Action Fund and NARAL Pro-Choice America – warned liberals not to dismiss Republican presidential front-runner Trump and called his campaign “a five-alarm fire for our democracy.”

“A hate-peddling bigot who openly incites violence is the likely presidential nominee of one of our nation’s two major parties,” they wrote. “It is alarming and dangerous.”

The letter was first reported by Politico.

“If the Republican Party nominates Donald Trump for president – and the odds are it will – there’s every reason to believe he has a serious shot at winning the general election. We cannot afford to underestimate him until it’s too late, as many Republicans now regret having done during the primaries,” the letter said. “If we wait to see how things shake out to make our plans, it’ll be too late and November will come sooner than anyone thinks.”

The Trump campaign did not immediately respond to a request for comment. But the Republican presidential front-runner said in an interview with CNN’s Jake Tapper on “State of the Union” Sunday that he “should get credit, not be scorned” for his handling of the tension at his events.

“You had professional disruptors, thousands of them, from Sanders and to a smaller extent, Hillary (Clinton),” he said of a Friday night event in Chicago that Trump canceled after chaos broke out there.

Trump said of conservatives: “We are treated so unfairly, and I’m treated very unfairly.”

He also specifically aimed a warning at Sanders, suggesting that his supporters could disrupt the Vermont senator and Democratic hopeful’s events after some of the Chicago protesters brought pro-Sanders signs.

The “massive mobilization” by progressives would have a three-part strategy composed of “non-violent mobilization and organizing” and “vigorous social media presence,” pressing media outlets to “condemn Trump’s racism, misogyny and xenophobia” and a “voting renaissance,” according to the letter.

The letter noted that the affiliations of the signatories “are shown for informational purposes only and do not imply endorsement by the organization.”

Though the letter does not represent formal endorsements – and the signatories lead groups committed to Democrats Hillary Clinton, Bernie Sanders or neither – the release signals a readiness among progressive groups to present a united front against Trump in a general election, regardless of the length or result the Democratic primary.

CNN’s Eric Bradner contributed to this report.