Story highlights Thousands of progressives gathered in Chicago

Anger at the Democratic party and its chairwoman are palpable

Chicago (CNN) Hillary Clinton may be the presumptive Democratic nominee, but the fight to unify the party and its traditional allies in the wake of an unexpectedly long and contentious primary is poised to go on much longer.

The more than 3,000 Bernie Sanders supporters and progressive activists gathered here at the "People's Summit" have engaged in little open talk about Clinton, preferring instead to plot a path forward in the wake of the Vermont senator's defeat -- and questioning the motivations of the Democratic Party and the legitimacy of its nominating contest.

"There is massive corruption in the machinery of the Democratic Party," said RoseAnn DeMoro, the executive director of National Nurses United, the powwow's principal organizer, who had endorsed Sanders. "The only way that we can overcome that corruption and manipulation is for all of us not to work in isolation."

DeMoro lamented the party's push to gather endorsements and support from Sanders' backers, calling it "a very negative dialogue."

Its message, she said, is that "if you're not with Clinton, then you're pro-Trump."

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