(CNN) On Tuesday afternoon, as Illinois Rep. Luis Gutierrez announced his retirement plans and endorsed Cook County Commissioner Chuy Garcia in the coming Democratic primary, the country's largest socialist organization was mobilizing behind a candidate of their own.

Chicago Alderman Carlos Ramirez-Rosa, a Democrat and member of the Democratic Socialists of America, jumped into the contest right alongside Garcia, kicking off a race that could vault a new crop of ambitious leftists -- DSA's dues paying membership jumped to more than 31,000 from about 5,000 over the past year -- into mainstream political prominence.

What comes next will be a step up in class — and a stress test for the growing movement. DSA-backed candidates scored a series of victories earlier this month, most notably in Virginia, where Lee Carter defeated Jackson Miller, the Republican majority whip in the House of Delegates. But Garcia, who supported Sen. Bernie Sanders early on in the 2016 Democratic primary cycle (after Sanders backed his bid to oust Chicago Mayor Rahm Emanuel), is a well-credentialed progressive.

"They're both really strong candidates, so what Carlos needs to do is articulate why his vision, and a vision for democratic socialism, really needs a voice in Washington that it lacks right now," said David Duhalde, DSA's deputy director. That means convincing voters that Ramirez-Rosa "would be better to represent not only the people from his district, but the people of the United States by representing a unique viewpoint that's sorely missing (in the House) since Sanders went to the Senate in 2006."

Overnight, as rumors spread that Gutierrez would reverse course and stand down, and Ramirez-Rosa told CNN he planned to "begin circulating (paperwork) tomorrow" if the seat came open, DSA members online reacted joyously -- "folks are pumped," Ramirez-Rosa noted drily in a text -- and began to bubble with plans to organize behind his campaign.

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