Two other Democrats — Setti Warren, the mayor of Newton, and Bob Massie, a onetime candidate for lieutenant governor — also tried to work the crowded Park Plaza ballroom. But Warren, the keynote speaker, was clearly the star of the day.

"I welcome Elizabeth Warren into the race," said Alan Khazei, looking anything but happy about the latest development in the 2012 campaign to defeat Republican Senator Scott Brown. Citing his 25 years of grassroots organizing, Khazei, the City Year co-founder, called the Democratic primary "wide open" and insisted he's in it to stay. Khazei said the competition will be good for Democrats and in the end, "We're gonna beat Scott Brown."

Warren, a Harvard professor and consumer advocate, has yet to officially announce plans to run for her party's nomination. But after Monday morning's debut before the Greater Boston Labor Council, her formal entrance seems like just a matter of time.

After a gushing introduction that likened her to the late, great Ted Kennedy, Elizabeth Warren gave a rousing speech that brought hardcore labor organizers to their feet — and left the already — announced Democratic candidates for Senate with stricken looks on their faces.

The great philosopher, Eminem, once mused " if you had one shot, or one opportunity... would you capture it, or just let it slip?" Warren captured it, by artfully mixing personal biography with a passionate call to battle against Wall Street. At one point, she also noted that General Electric pays no taxes "while you and I do". It could be interpreted as a sly shot at President Obama, who appointed GE CEO Jeffrey Immelt as head of the President's Coucnil on Jobs and Competitiveness — while deciding not to nominate Warren to head the new consumer protection agency that she set up at Obama's behest.

Asked afterward about Warren's performance, state Treasurer Steven Grossman said, "This is a tough audience. I thought she did very well. She had the rapt attention of the audience. She connected."

Given positive reviews from this appearance and Warren's previous meet-ups with liberal activists across the state, the bandwagon effect will be hard for Democrats to resist. Because of the $1 million he has already raised, Khazei was considered the strongest challenger in a generally weak field. There will be pressure for him and the others to drop out, and clear the way for Warren to quickly make it a one-on-one fight against Brown.

As for Warren's chances against Brown, one speech does not win a primary — or a general election. But rallying labor to her side is important. In the last Senate special election, the Massachusetts AFL-CIO unanimously endorsed Martha Coakley. But on election day, exit polling showed that Brown won more support from labor households. It did not take long for the new senator to squander that support. The first vote he took on the Senate floor was to block Obama's pick for the National Labor Relations Board.