BOSTON — Marking a year since she began her campaign for president and seeking to frame the final five-week sprint to Iowa where much of her candidacy hinges, Senator Elizabeth Warren urged her supporters to “imagine that something better lies on the other side of the chaos and ugliness of the last three years” in a major speech on Tuesday.

Ms. Warren, who surged to the front of the Democratic field in early fall but has since receded, warned her party against “thinking small” in 2020 as she pitched her sweeping agenda. “Americans do big things,” she said. “That’s who we are.”

Speaking at the Old South Meeting House in Boston, where revolutionaries plotted and organized against the British in the 1770s and planted the seeds of the American Revolution, the senior Massachusetts senator asked her supporters — and would-be supporters — to envision a new America under a President Warren.

“Today we come together to imagine,” Ms. Warren said, using variations of “imagine” roughly 50 times as she ticked off plans for providing universal health care, wiping away student loan debt, tackling climate change, addressing gender and racial inequities and curbing the power of money in politics.