Senator Elizabeth Warren of Massachusetts brought her ascendant presidential campaign to New York City on Monday night, unspooling a forceful argument for attacking corruption in government in a defining speech of her White House bid.

Addressing thousands of supporters in Washington Square Park, some holding up “I’m a Warren Democrat” signs, Ms. Warren pressed her case to bring sweeping change to an economic and political system she views as fundamentally tilted to favor the wealthy and powerful.

She spoke near the site of the Triangle shirtwaist factory fire of 1911, which killed 146 garment workers, most of them women. The fire spurred a push to improve workplace safety, which Ms. Warren harnessed as a parallel for the far-reaching change she wants to pursue as president.

And once again, she urged Democrats to embrace her call for fundamental change — not the kind of incremental approach favored most notably by Joseph R. Biden Jr., the former vice president and the primary race’s front-runner.

“There’s a lot at stake in this election, and I know people are scared,” Ms. Warren said from a lectern in front of the park’s marble arch. “But we can’t choose a candidate we don’t believe in just because we’re too scared to do anything else. And Democrats can’t win if we’re scared and looking backward.”

Ms. Warren was repeatedly met with enthusiastic cheers, and the crowd broke into chants of “two cents” when Ms. Warren laid out her case to impose a 2 percent tax on the fortunes of the super rich.

Ms. Warren was introduced by Maurice Mitchell, the national director of the Working Families Party, which endorsed Ms. Warren earlier Monday.

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