Four years ago, Democrats allotted Representative Alexandria Ocasio-Cortez of New York a scant 90 seconds to speak at their convention. She used it to symbolically nominate Senator Bernie Sanders for president and never mentioned Joseph R. Biden Jr.’s name.

So when Ms. Ocasio-Cortez took the convention stage on Monday night in Chicago shortly before Hillary Clinton, her prime-time speaking slot offered a vivid display of how far the Democratic Party and the leader of its progressive wing have moved to embrace each other since 2020.

Greeted with chants of “A-O-C,” Ms. Ocasio-Cortez, a democratic socialist who made her name by taking on the Democratic establishment, delivered an affectionate tribute to Mr. Biden, laced into Donald J. Trump and forcefully endorsed Vice President Kamala Harris as a champion of working Americans.

“We know Trump would sell this country for a dollar if it meant lining his own pockets and greasing palms of his Wall Street friends,” Ms. Ocasio-Cortez said. “And I, for one, am tired of hearing about how a two-bit union buster thinks of himself as more of a patriot than the woman who fights every single day to lift working people out from under the boots of greed trampling on our way of life.”

She added: “The truth is, Don, you cannot love this country if you only fight for the wealthy and big business.”

The thunderous applause that followed would have been unthinkable only a few years ago. At their last convention, Democrats seemed more comfortable spotlighting Republicans supporting Mr. Biden than a young leftist like Ms. Ocasio-Cortez, whose policies and rhetoric they feared would alienate moderate swing voters.

Since then, though, she has worked closely with the White House on economic and climate policy, served as a Biden surrogate and increasingly won over allies in Democratic leadership — sometimes at the expense of leftist support.

Her allies say the shift reflects Ms. Ocasio-Cortez’s pragmatic streak as well as the party’s growing willingness to embrace its left flank. An aide to the congresswoman said the party had proactively invited her to speak at the convention and had made no effort to edit or temper her remarks.

For her part, Ms. Ocasio-Cortez, 34, largely sidestepped painful fissures with her party over the war in Gaza that have cast a shadow over the convention. The congresswoman has been a vocal critic of the war, which has killed tens of thousands of Palestinians over the past 10 months, and has led calls to cut off American military aid to Israel. But, even as pro-Palestinian demonstrators gathered in Chicago to protest outside the convention, she conspicuously credited Ms. Harris with “working tirelessly to secure a cease-fire in Gaza and bring the hostages home.”

Instead, Ms. Ocasio-Cortez focused her attention on working-class voters. She leaned on her own biography, reminding viewers that only “six years ago, I was taking omelet orders as a waitress in New York City.” Ms. Harris, she said, rose from a similar background.

“To love this country is to fight for its people — all people, working people, everyday Americans like bartenders and factory workers and fast-food cashiers who punch a clock and are on their feet all day in some of the toughest jobs out there,” the congresswoman said.