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.

Rep. Tulsi Gabbard (D-Hawaii) predicted Monday that it would be more difficult for House Democrats to remain in control of the House following passage of articles of impeachment against President Trump.

In a video tweeted Monday evening, the 2020 candidate for president wrote that Trump's chances of winning reelection had been "greatly increased" because of the House's vote.

"Unfortunately, the House impeachment of the president has greatly increased the likelihood Trump will remain the president for the next 5 years," Gabbard says in the video.

"We all know that Trump is not going to be found guilty by the U.S. Senate," she added.

Liberals swooned. The Democratic establishment was on its heels. Powered by small donors and grassroots energy, in late 2003 Iowa seemed Howard Dean’s for the taking.

But then came the attacks — from both the center and the left. The surge became a stall. On caucus day, the Vermont governor came in third and never recovered.

Elizabeth Warren is trying to avoid the same fate.

While the Warren campaign has stuck to its corruption-centric message, it has noticeably shifted tactics in recent weeks. The Massachusetts senator changed her stump speech and is now taking many more audience questions. She’s drawing sharper contrasts with opponents after months of trying to stay above the fray. There are more big, thematic campaign speeches, and more off the record chats with reporters.

The third such speech in recent weeks will be Tuesday in Boston at the Old South Meeting House, a key organizing place for the Boston Tea Party and American revolutionaries in the 18th century. Warren will highlight the work and poetry of Phillis Wheatley — a member of the Meeting House's congregation. Wheatley was kidnapped from Africa and enslaved before becoming one of the first black women poets in America.

SOUTH BEND — Pete Buttigieg stood before the Common Council Monday night for his final time as mayor, delivering a farewell address that served as a detailed defense of his record, especially in addressing the city’s toughest problems and struggling neighborhoods.

“South Bend’s trajectory has been transformed,” he declared at the beginning of speech. He later ended with another declaration, while noting many challenges remain: “South Bend is back.”

In spelling out the accomplishments during his eight years as mayor, Buttigieg called for his friend and successor, James Mueller, to build on work in the areas of public safety, economic development and housing.

Buttigieg also addressed his personal life in South Bend, saying he was not sure there was a place for him in his hometown after leaving for college.