Washington: The US Democratic Party was rocked to its core Tuesday when establishment congressman Joe Crowley — seen as a potential standard bearer — was ousted by a 28-year-old far-left political novice.

Crowley, a 10-term incumbent representing New York's 14th district, and the fourth most powerful Democrat in the House of Representatives, had been considered a potential replacement for Nancy Pelosi as speaker should Democrats reclaim the majority in November's midterm elections.

Instead, the 56-year-old Crowley, chairman of the Democratic caucus, suffered the biggest shock upset to date in the 2018 political season, losing his primary to Alexandria Ocasio-Cortez, a Bronx-born Latina activist and Bernie Sanders supporter who has never held elected office.

According to a detailed profile in Vogue magazine, just months ago Ocasio-Cortez was still working as a server in a restaurant.

Her Facebook page highlights her political positions — expanded Medicare for all, tuition-free college, and criminal justice reform — that are similar to those put forward by Sanders, a Vermont socialist who ran for the Democratic presidential nomination against Hillary Clinton in 2016.

The implications for Ocasio-Cortez's primary victory are profound.

It highlights the party's surging progressive wing, as it grapples with its identity in the era of Republican President Donald Trump, and in the run up to November's midterm elections when Democrats are aiming to reclaim the House.

Trump swiftly weighed in on Twitter.

"Wow! Big Trump Hater Congressman Joe Crowley, who many expected was going to take Nancy Pelosi's place, just LOST his primary election. In other words, he’s out!" Trump said.

"That is a big one that nobody saw happening.