Congresswoman Alexandria Ocasio-Cortez is set to back Bernie Sanders as the Democrat to take on Donald Trump in the 2020 presidential election.
Ms Ocasio-Cortez is the youngest ever US congresswoman and has become a leading progressive voice in the party.
She is scheduled to appear at a “Bernie’s Back” rally in New York to formally offer her support on Saturday.
Mr Sanders, 78, took part in a debate on Tuesday night, less than two weeks after suffering a heart attack.
He took the opportunity to try and ease concerns over his health during the appearance, telling the debate crowd in Ohio: “I’m healthy, I’m feeling great.”
Mr Sanders is currently third in the polls to win the party’s nomination, behind former Vice-President Joe Biden and Senator Elizabeth Warren.
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Mr Sanders previously ran for the Democratic nomination in 2016, but lost to Hillary Clinton.
An endorsement by Ms Ocasio-Cortez is regarded as a significant boost for Mr Sanders’ campaign and a big blow for fellow progressive candidate Elizabeth Warren.
Ms Ocasio-Cortez has become a high-profile voice within the Democratic Party since she beat a political veteran congressman for his seat in a primary vote 2018.
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The attention has also made her a frequent target of political conservatives, including President Trump himself.
Earlier this year Mr Trump made headlines when he told Ms Ocasio-Cortez and three other members of a group of liberal Congresswomen of colour to “go home” in comments condemned widely as racist.
On Tuesday another member of the group, informally known as “the squad”, formally offered her support to Mr Sanders.
Ilhan Omar praised him for leading a “working class movement to defeat Donald Trump that transcends generation, ethnicity, and geography”.
The other two “squad” members, Congresswomen Rashida Tlaib and Ayanna Pressley, also have yet to declare. CNN indicates that they too will back Mr Sanders.
The independent senator from Vermont has so far raised more than $60m in individual campaign contributions – more than any other Democrat in the running to become president – according to New York Times analysis.
Before Ms Ocasio-Cortez entered politics herself, she did local grassroots volunteering Mr Sanders’ 2016 campaign.
The 78-year-old shares stances with Ms Ocasio-Cortez on a number of key issues, including an ambitious plan to tackle climate change and raising the federal minimum wage to $15.
If he wins in 2020, Mr Sanders has pledged to increase taxes on the extremely wealthy and introduce free university education and universal healthcare coverage to all Americans.
Despite being the oldest candidate in the race, the senator gained a reputation in 2016 for his ability to appeal to younger voters.
The trend looks set to continue and earlier this year he sat down with 26-year-old rapper Cardi B to discuss politics for a campaign video.
Who will take on Trump in 2020?
Election day is still more than a year away but the race to become the Democratic challenger to Mr Trump is already well under way.
The latest polling suggests Ms Warren and Mr Biden are the front-runners, while Mr Sanders is also a popular contender. Many of the the other candidates are relatively unknown outside the Washington DC bubble.
Find out more about the contenders