BOSTON — Harvard Law School professor Elizabeth Warren offered a preview of her likely Senate campaign stump speech Monday, presenting herself to hundreds of union officials, activists and local politicians at a Labor Day breakfast as a champion of the middle class.

“The middle class has been hacked at, chipped at and hammered for a generation,” Warren said to a packed ballroom at the Park Plaza hotel. “We can’t take it much longer.”

Her pitch to the crowd of union faithfuls was that her life had been and would be devoted to continuing to stand up “to powerful interests” in defense of the middle class.

Warren, who was in charge of creating the Consumer Financial Protection Bureau until August, told the crowd here at the Greater Boston Labor Council’s annual breakfast that she is frustrated by a Washington that “wasn’t working for us” and saw the America where children had a chance to do better than their parents slipping away.

The language is remarkably similar to what President Barack Obama used on the campaign trail during his presidential run.

“I’m not giving up without a fight,” Warren said, poised to officially enter the Democratic primary race for the nod to challenge Republican Sen. Scott Brown.

“I’m going to keep fighting for middle-class families, for working people. Whether I fight as an outsider or I fight from the floor of the Senate —” Warren said before her words were halted due to a raucous and lengthy standing ovation.

Finally, the applause died down, and Warren said, “This is a big fight — the biggest fight of our lifetimes — and we will throw everything we’ve got into it.

“We cannot afford to lose. We will not lose!” she said to the audience.

The speech, anchored in her early life, stressed her biography. She told activists that she grew up on the “ragged edge of the middle class.”

“I know the anxiety that comes with living one pink slip or one serious medical problem away from financial collapse,” Warren said. “I know it because I’ve lived it.”

Only making a glancing reference to her tenure at Harvard during the 20 minutes she spoke, Warren appeared to be working to dispel any narrative that her connection to the nation’s oldest and most prestigious university makes her an elitist. Warren is teaching a course on contract law Monday and Tuesday mornings from 10 a.m. to noon.

Discussing infrastructure investment, she also struck a few populist notes, noting the incongruity of General Electric avoiding paying any taxes while middle-class Americans are taxed on “every dollar.”

“Instead of subsidizing the past — those who have already made it — we need to invest in the future,” she said to applause.

The optics of the event made local unions’ likely choice of primary candidates clear. While declared Senate hopefuls Newton Mayor Setti Warren, CityYear co-founder Alan Khazei and activist Bob Massie attempted to work the periphery of the large ballroom, Warren — the keynote speaker of the event — sat on the stage next to Boston Mayor Tom Menino (D).

Menino and Warren chatted amicably on dais in the minutes before the event began, but he appeared to be a long way from endorsing Warren’s candidacy.

Menino told the Boston Globe last week he understands Brown’s appeal to voters. And as for Warren, it sounded like he still had his doubts.

“You know, the media can’t make you,” Menino told the Globe. “You have to be saleable. Do I know she can be saleable? I don’t know that.”