Senator Elizabeth Warren, the Massachusetts Democrat and a sharp critic of big banks and unregulated capitalism, entered the 2020 race for president on Monday, becoming the first major candidate in what is likely to be a long and crowded primary marked by ideological and generational divisions in a Democratic Party desperate to beat President Trump.

In an 8:30 a.m. email to supporters on New Year’s Eve — 13 months before the first votes will be cast in the Iowa caucuses — Ms. Warren said she was forming an exploratory committee, which allows her to raise money and fill staff positions before a formal kickoff of her presidential bid. Ms. Warren also released a video that leaned on the anti-Wall Street themes, a campaign message that drew strong praise Monday morning from liberal groups.

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“I’ve spent my career getting to the bottom of why America’s promise works for some families, but others, who work just as hard, slip through the cracks into disaster,” Ms. Warren said in the video. “And what I’ve found is terrifying: these aren’t cracks families are falling into, they’re traps. America’s middle class is under attack.”

“But this dark path doesn’t have to be our future,” she continued. “We can make our democracy work for all of us. We can make our economy work for all of us.”