“They work for the rich and the powerful and not the rest of us,” Ms Warren said of lobbyists who seek to influence legislators, in a speech to more than 500 people at a town hall.
The crowd cheered as she warmed to her theme.
“This is corruption, pure and simple,” she said. “It is corruption and it is eating away at our democracy and every fibre of our lives.”
Ms Warren’s words built on ideas she trumpeted in a video released on New Year’s Eve, when she announced that she had formed an exploratory committee to run for the White House.
Iowa’s caucuses remain hugely influential in presidential elections, as its residents are the first to vote in primaries.
The results are the first indicator of the strength of a prospective presidential candidate and Ms Warren was greeted with enthusiasm by crowds in Sioux City, Des Moines and Council Bluffs.
The former law professor, a leading Senate Democrat, alluded to wealthy potential Democratic rivals Tom Steyer and Michael Bloomberg, by arguing for elections free from the influence of billionaires.
“None of us believe that billionaires ought to be able to self-fund [their campaigns],” she said, during a day of public speeches and private meetings.
Mr Steyer is expected to visit Iowa later this week.
“We need big structural change,” Ms Warren said. “We’ve got to go big on this.”
The senator made her name as an advocate for consumer protection and has frequently clashed with Donald Trump, who has derided her in speeches and online.
She was questioned at one event about her decision to take a DNA test after the president questioned her claim of Native American ancestry.
The test results suggested Warren could have had a Native American ancestor many generations ago, but the senator was widely criticised for her decision.
Ms Warren said that she had never claimed to be a member of a Native American tribe and returned to her message of anti-corruption.
After watching the politician in action, retired Sioux City teacher Colleen Sernett-Shadl said: “She comes across as real. And I like that. I just want to know whoever we nominate is strong. And there are a lot of people to sift through.”
Ms Warren said that she wanted to run a grassroots campaign and urged audience members to donate money in small amounts.
The Iowa caucuses will officially begin the 2020 nominating campaign in February 2020 and Ms Warren’s visit is part of an effort to gain an early advantage in the state.
Other Democratic presidential prospects are expected to announce their plans in the coming weeks and many have been in touch for weeks with party leaders, activists and potential staff in Iowa.
Additional reporting by agencies
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