“All of us, together, raising our voices — that’s what’s going to make real change,” she said. “And Iowa is going to have a big part of determining where we go next.”

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The event not only kicked off Ms. Warren’s campaign, but marked the starting point to what figures to be a long and grueling Democratic primary season. Dozens of candidates, including governors, senators and big-city mayors — many of whom will be new to the national stage — are considering joining the race in the coming months.

Some have characterized the 2020 Democratic nomination process as perhaps the most wide open since 1992: The party has no single leader, no obvious front-runner to challenge President Trump in 2020, and no broadly unifying ideology as it moves away from a quarter-century of dominance by the Clintons and Barack Obama.

Ms. Warren, who will be among the top-tier candidates no matter who else runs, has staked out a position as one of the party’s most resolute voices on the left. Before she took questions from the audience, she laid out her overarching premise of America’s ills: too much corporate money in the political system, a retrenchment from government regulation in the market and even overt discrimination that has affected racial minorities.

“It is an America right now who works for the rich and powerful. And we need to call it out for what it is: corruption pure and simple,” Ms. Warren said. “What’s happening to opportunity in this country? Why is the path so rocky for many — and why is it so much rockier for people of color?”