She was open, when asked, about her affection and appreciation for both Democratic candidates. A decision, if it was going to come at all, wasn't expected until later this year.
"I would like to see in a presidential candidate is one that has a coherent worldview and logic from which all these policy proposals are coming forward," Ocasio-Cortez told CNN in May. "I think Sen. Sanders has that. I also think Sen. Warren has that. I also want to see us centering (on) working people in the United States to stem income inequality (and) tackle climate change."
But as the primary heated up, with Warren surging and Sanders stalling in the polls, the process appeared to be speeding up.
On a late September weekend, the New York Democrat visited Burlington, Vermont, where she, Sanders and a couple of close aides met for dinner, according to a source familiar with discussions. They met against the next day for brunch. Sanders campaign manager Faiz Shakir later told CNN those discussions did not yield any firm commitment but had been "a key step in the process."
Ocasio-Cortez's endorsement was one of the most sought-after in progressive politics and, ultimately, she delivered it at one of the most uncertain times in Sanders' long political career.
The New York Democrat told Sanders she would back him for president over the phone as he was lying in a hospital bed recovering from the heart attack that took him off the campaign trail for weeks, aides to both told CNN. When he fell ill in Las Vegas, Sanders' campaign had been stalling in polls, as Warren pulled ahead and solidified her status among the front-runners.
On Saturday, the influential freshman who rose quickly to become one of the most prominent progressive voices in Congress injected a fresh burst of excitement into the Sanders campaign at a big rally in Queensbridge Park.
What it means for Sanders' campaign is an open question. What it meant to him and his supporters, is difficult to overstate.
Addressing thousands of supporters, Ocasio-Cortez told the story of how "Tío Bernie," or Uncle Bernie, as she called him, made her believe she — at the time a "sexually harassed waitress" in downtown Manhattan — could push for something more, and eventually run for the House seat she won last year.
"The only reason that I had any hope in launching a long-shot campaign for Congress is because Bernie Sanders proved that you can run a grassroots campaign and win in an America where we almost thought it was impossible," Ocasio-Cortez said.
In her old job, Ocasio-Cortez said she was like so many other service workers logging 12-hour days with no structured breaks.
"I didn't have health care. I wasn't being paid a living wage," Ocasio-Cortez said. "And I didn't think that I deserved any of those things."
Then something changed.
"It wasn't until I heard of a man by the name of Bernie Sanders that I began to question and assert and recognize my inherent value as a human being that deserves health care, housing, education and a living wage," she said.
Ocasio-Cortez credited Sanders for what she described as one of the best Democratic presidential primary fields in a generation and touted his influence on the freshman Democratic congressional class.
She cited his rejection of corporate PAC money, his championing of "Medicare for All" and his urgent support for the Green New Deal in the face of a global climate crisis.
Ocasio-Cortez said the pressure members of Congress come under is "no joke," and praised Sanders for standing up to big corporations and establishment interests.
And Sanders' advocacy, which she described as "enormous, consistent and nonstop," inspired her, and the movement that has grown up around them both, to back him — and their fight.
Ocasio-Cortez told CBS News on Saturday endorsing Sanders was "the most authentic decision to let people know how I feel and where I am," and said it was not based on a political calculation.
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