New York (CNN) Sen. Bernie Sanders returned to the campaign trail in New York on Saturday for a rally with Rep. Alexandria Ocasio-Cortez, who delivered to him one of the most sought-after endorsements in progressive politics and called for a "mass mobilization" of young, working class voters in 2020.

"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 who deserves health care, housing, education and a living wage," Ocasio-Cortez said, as she traced her trailblazing path from "sexually harassed waitress" across the river, in downtown Manhattan, to the halls of the United States Congress.

The "Bernie's Back" rally in Queensbridge Park, just outside the border of the congressional district Ocasio-Cortez was elected to represent last November, comes days after word that she and "Squad" ally Democratic Rep. Ilhan Omar of Minnesota would endorse Sanders. Democratic Rep. Rashida Tlaib of Michigan is expected to join them soon, potentially at an event later this month in her home district.

Speaking in the shadow of the Ravenswood Generating Station smokestacks, Ocasio-Cortez pointed to the site of the event as "ground zero" in the fight for "environmental justice" and, with the Queensbridge Houses, the largest development of its kind in the United States, to her left, public housing in America. The proximity of the plant to the apartment buildings, Sanders said later, was an example of the "environmental racism" his Green New Deal is designed to combat.

In her first public remarks since news of her endorsement spread, Ocasio-Cortez also praised the current field of Democratic presidential candidates as among the most progressive the party has ever seen -- and credited Sanders for paving the way. He returned the favor minutes later, as the pair clasped hands and raised their arms in triumph.

"I've been around politics a few years and it is hard to believe the degree to which in less than one year, this woman, the youngest woman ever elected to Congress, it is hard to believe the degree to which she has transformed politics in America," Sanders said, praising Ocasio-Cortez as "a fierce defender of the working class of our country" and describing her as "an inspiration to millions of young people, not just here in New York, but across this country, who now understand the importance of political participation and standing up for justice."

Ocasio-Cortez told Sanders she would back him in a phone call while he was still in a Nevada hospital after a heart attack less than three weeks ago, aides to both confirmed, and news of her decision has been greeted as a welcome -- and validating -- shot of adrenaline inside a campaign that has stalled in most polling and was rattled, many on a very personal level, by Sanders' heart attack.

'I am more than ready'

Sanders and the surrogates who joined him made a point of hitting back against suggestions that he might be too old, or insufficiently fit, to win the White House next year. The 78-year-old Vermont independent was, per usual, particularly direct.

"I am happy to report to you that I am more than ready, more ready than ever to carry on with you the epic struggle that we have today," Sanders, who came onstage to AC/DC's "Back in Black," said. "I am more than ready to assume the office of President of the United States. I am more than ready to take on the greed and corruption of the corporate elites and their apologists ... To put it bluntly, I am back."

A diverse crowd of Sanders supporters began to fill a stretch of the park in Northwest Queens, a hub of the progressive activism that helped scuttle Amazon's plans to build a new headquarters, here in Long Island City, hours before Sanders and Ocasio-Cortez took the stage.

Sanders' wife, Jane, declared her husband "healthy" and "more than ready to continue his life-long struggle to fight for the working people of America. She was followed at the mic by filmmaker Michael Moore, who endorsed Sanders while scorning media coverage of the campaign.

"The only heart attack we should be talking about," Moore blared, "is the one that Wall Street is going to have when Bernie Sanders is president."

Sanders made his in-person return to the primary stage at Tuesday's CNN/The New York Times debate in Ohio . While Elizabeth Warren of Massachusetts, his fellow Senate progressive and leader in a number of recent polls, seemed to be at the center of every skirmish and the focal point of increasingly blunt attacks from the contest's centrist candidates, Sanders delivered one of his sharpest debate performances -- a showing that, at least for now, has seemed to quiet concerns over his health.

Saturday's rally, though, was something entirely different -- as Ocasio-Cortez, who had Sanders and Ocasio-Cortez have campaigned together before, during the summer of 2018, for progressive Democratic primary candidates.Saturday's rally, though, was something entirely different -- as Ocasio-Cortez, who had publicly weighed endorsing either Sanders or Warren, committed herself to his second crusade for the Democratic nomination.

Her endorsement underscored an argument the campaign has been making for months, which contends that Sanders is uniquely positioned to forge, as his campaign manager, Faiz Shakir, described it earlier this week, "the multiracial working class coalition we need to win."

Ari Rabin-Havt, Sanders' chief of staff, told CNN after the debate -- just a few hours after The Washington Post first reported on the endorsement -- that Ocasio-Cortez's decision would give new fuel to the "movement" that Sanders hopes will sweep him into the White House.

"This isn't about Bernie Sanders. This is about us," Rabin-Havt said. "And this is what these endorsements are about. These are people who want a movement for change in this country. A movement for 'Medicare for All,' a movement for a Green New Deal, a movement on these issues. And you know, when Bernie Sanders says, 'Not me, us,' I think that message resonates with members of Congress who believe strongly in a movement."

The endorsement, Justice Democrats Executive Director Alexandra Rojas said on the eve of the New York rally, is "significant in the sense that it shows that the Bernie campaign is important to progressives and Democratic voters."

"I know that Justice Democrats and myself (in this role) wouldn't even exist if it hadn't been for Bernie's 2016 presidential run," Rojas said. "He has a young, racially diverse, totally working-class base of support, which is the same people that Alexandria Ocasio-Cortez represents. She looks so much more like the grassroots base of the Democratic Party than Chuck Schumer, than Nancy Pelosi. And I think that's why the endorsement was so important for Bernie."

Ocasio-Cortez had indicated in previous months that she could hold off in choosing a presidential candidate to support until later in the year, or even into the primaries, which begin in February with the Iowa caucuses. But rumors that she might get involved sooner heated up after a late September visit to Burlington, Vermont, where she, Sanders and a couple of close aides met for dinner, according to a source familiar with discussions, and then again the next day for brunch.

Shakir said Tuesday night that those conversations, while they did not lead to any kind of firm commitment, were "a very key step in the process."

"And then we, obviously, our teams worked together on a lot of details," Shakir said, referencing a rollout that had begun earlier that day when the campaign began teasing the appearance of a "special guest" at the New York rally.

Less than a week later, with Ocasio-Cortez watching on, Sanders looked ahead: "I look forward," he said, "to traveling with her all over this country."

This story has been updated with additional developments Saturday.