Rep. Joe Crowley, the chairman of the House Democratic Caucus who was thought by some to be a future Speaker of the House, suffered a shocking primary defeat in New York's 14th House District Tuesday.

With 88 percent of precincts reporting, 28-year-old Alexandria Ocasio-Cortez -- who has never held elected office -- led Crowley by more than 3,600 votes.

Ocasio-Cortez, a Bernie Sanders supporter who has called for the abolition of U.S. Immigration and Customs Enforcement (ICE), had gained the endorsement of several left-wing groups, including MoveOn and the Democratic Socialists of America (DSA).

"We have built power. We have organized," Ocasio-Cortez said on Twitter as the polls closed. "What we have built is permanent. No. Matter. What."

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Soon after The Associated Press called the race, the New York City branch of the DSA tweeted congratulations to Ocasio-Cortez, saying that her victory proved "that working class people are hungry for a voice in politics."

In a statement, Crowley congratulated Ocasio-Cortez on her victory and said he looked forward to supporting her against Republican Anthony Pappas in November.

"The Trump administration is a threat to everything we stand for here in Queens and the Bronx, and if we don't win back the House this November, we will lose the nation we love," Crowley said. "This is why we must come together. We will only be able to stop Donald Trump and the Republican Congress by working together, as a united Democratic Party."

The National Republican Campaign Committee celebrated the defeat of "poor Joe Crowley."

"House Democrats, hoping for a post-Pelsoi era, are now left leaderless," NRCC spokesman Matt Gorman said. "The only person happier tonight than Nancy Pelosi is the NRCC."

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President Trump also reveled in the downfall of the man he called "Big Trump Hater Congressman Joe Crowley"

"That is a big one that nobody saw happening," Trump said of Crowley's defeat. "Perhaps he should have been nicer, and more respectful, to his President!"

Crowley, the fourth-ranked House Democrat and a ten-term incumbent, was viewed by many observers as the one person a broad enough support base to potentially succeed House Democratic Leader Nancy Pelosi or House Democratic Whip Steny Hoyer. His defeat leaves a large gap in the House Democratic leadership and ensures that Pelosi and Hoyer will be able to remain in their posts, if they choose to do so.

In a statement, Pelosi described Crowley as "an unwavering champion for America’s working families for almost two decades" who "brought principled, unifying and forward-looking leadership to the historic challenges of the Trump Administration" as the Democratic caucus chair.

"I salute Chairman Crowley for a formidable legacy of achievement for the people of New York," Pelosi concluded. "I congratulate Alexandria Ocasio-Cortez on her victory."

Fox News' Chad Pergram and The Associated Press contributed to this report.