The Democratic presidential candidates expressed uniform support Wednesday night for the impeachment inquiry into President Donald Trump’s conduct toward Ukraine but signaled significant reservations in the fifth primary debate about the possibility that the 2020 campaign could become submerged in a congressional investigation of Trump’s behavior.

The debate in Atlanta began with few sparks between the leading candidates but a barrage of fire directed at Trump and what the top Democrats described as a culture of corruption and self-dealing in his administration. That line of argument crossed ideological and cultural lines on the Democratic side, involving populist liberals like Sens. Elizabeth Warren of Massachusetts and Bernie Sanders of Vermont and their more moderate competitors, including Sen. Amy Klobuchar of Minnesota and Mayor Pete Buttigieg of South Bend, Indiana.

“We cannot simply be consumed by Donald Trump, because if we are, you know what? We’re going to lose the election,” Sanders said, pointing to social problems like homelessness and climate change, which he termed “the great existential crisis of our time.”

For at least the first hour, the debate proceeded as a relatively subdued affair, with the candidates seeming to retreat from the harshest and most personal rivalries that flared a month ago at a debate in Ohio. Then, a gang of moderates — former Vice President Joe Biden, Buttigieg and Klobuchar — teamed up to savage Warren for her position on single-payer health care.

This time, candidates almost entirely refrained from clashing directly, framing their disagreements in polite or passive-aggressive terms, perhaps deliberately conceding that their intraparty competition could not compete for attention with sobering developments in the impeachment process.

But the seeds of a larger debate over policy and political strategy became evident even in their answers targeting Trump. Sanders and Warren railed against corruption in Washington, while other candidates, including Buttigieg and Biden, emphasized the importance of forging political unity and electing Democratic senators from red states.

Biden, who has staked his campaign on the perception that he is a strong general election candidate, used his leadoff answer to urge Democratic voters to pick a nominee who can “go into states like Georgia and North Carolina and other places and get a Senate majority.”

It was, atypically, Sen. Cory Booker of New Jersey, normally a nonconfrontational voice on the debate stage, who opened a more contentious phase of the debate, critiquing Warren’s proposal for a tax on the nation’s largest fortunes. “It’s cumbersome,” Booker jabbed. “It’s been tried by other nations. It’s hard to evaluate.”

Buttigieg soon followed suit, though, arguing that most Americans were on their side — but warning that Democrats must “galvanize not polarize that majority.”

Sanders made the case for his “Medicare for All” legislation and took an oblique shot at Warren, who has not prioritized the measure, noting that he would introduce his single-payer “in the first week” of his presidency.

But reflecting his de facto truce with Warren, Sanders reserved his sharpest words for the more moderate candidates who oppose Medicare for All, whom he did not name but whom he described as believing “that we should not take on the insurance industry, we should not take on the pharmaceutical industry.”

The exchanges grew notably less polite, though, when Sen. Kamala Harris of California was offered a chance to respond on a foreign policy question that went to Rep. Tulsi Gabbard of Hawaii, who targeted Harris at a debate over the summer.

Harris, with payback in mind, ignored the policy element of the question and unloaded on Gabbard. She noted that her rival has been a frequent guest on Fox News, refuses to call Syrian President Bashar Assad “a war criminal” and “buddied up to Steve Bannon” during Trump’s presidential transition.

Gabbard responded that Harris was merely offering “lies and smears and innuendo” and asserted that the senator would continue the “Bush-Clinton-Trump foreign policy of regime-change wars.”

Buttigieg, who is rising rapidly in the polls in Iowa, found himself for the first time in the position of defending his qualifications for the presidency, as the mayor of a small city who has never held statewide office. He cast skepticism of those credentials as a view from “traditional establishment Washington” and argued that from the vantage point of South Bend “the usual way of doing business in Washington is what looks small.”

Two of Buttigieg’s rivals pushed back more or less gently, channeling in a tentative way the frustration across the Democratic field with the rise of a 37-year-old mayor with no experience in national government.

Booker, a onetime wunderkind mayor of Newark, described himself as “the other Rhodes scholar mayor on this stage” — perhaps his most pointed expression of feeling overlooked in the race.

But it was Klobuchar who most effectively pivoted from her past criticism of Buttigieg, whom she has described as benefiting in the race from being male, into a forceful plea to the country to elect a female president.

“Women are held to a higher standard, otherwise we could play a game called name your favorite woman president,” Klobuchar said, brandishing one of her favorite lines from the campaign trail: “If you think a woman can’t beat Donald Trump, Nancy Pelosi does it every single day.”

The Democrats met just hours after the administration’s ambassador to the European Union, Gordon D. Sondland, offered perhaps the most damaging testimony against Trump yet in the House impeachment proceedings. The inquiry, centering on whether Trump linked U.S. financial and political support for Ukraine to a promise to investigate Biden, has worried some Democrats about the former vice president’s viability in a general election.

Yet that is only one factor that is making an already volatile race more fluid than ever.

Since the debate last month, former Gov. Deval Patrick of Massachusetts has entered the primary contest, and former Mayor Michael Bloomberg of New York has taken steps to do the same. They have been lured into the campaign in part over their concerns about the leftward drift in the party and also because there is still no clear front-runner with a little more than two months until the Iowa caucuses. It remains far from certain that they will be able to catch on so late in the race, but both have made clear that they are trying to win over more moderate voters.

If there is an opening for them, it is because Biden has not been able to consolidate support from center-left Democrats. Voters and donors from this wing of the party are uneasy with him mainly because of his lackluster fundraising and campaign performances, and less because of Trump’s false claims that he acted improperly with Ukraine when his son Hunter was on the payroll of an energy company there.

More broadly, though, the race remains unsettled because Democratic voters are splintered across racial, ideological and generational lines. Buttigieg surged in Iowa and New Hampshire in recent weeks, taking the lead in a new Des Moines Register-CNN survey of Iowa caucusgoers. But he has not made similar gains beyond the two heavily white states that kick off the nominating process.

The two leading populists in the field, Warren and Sanders, also are in the top tier in some early-state surveys thanks to strong support from self-identified liberals and many younger voters. But both have been unable to broaden their appeal to moderate Democrats. Together, these fractures have effectively created a four-way race, with none of the leading contenders yet proving they can break out of their demographic niche.

Warren had shown the most progress in expanding her coalition beyond white liberals, but she has endured a rocky stretch since coming under attack in the last debate over her failure to outline how she would pay for “Medicare for All.”

Last week she sought to tamp down criticism by unveiling a proposal for how to pay for her single-payer plan. But that seemed only to tie her more closely to an issue — replacing private health insurance with a government-run system — that many Democrats fear will hurt their chances in the general election. Warren has seen her polling dip in the past month, most notably in Iowa, but she retains a committed bloc of supporters.

It is a coalition that could be even larger were it not for Sanders’ revival. Polls show that many supporters of the two candidates view the other as their preferred second choice.

Sanders, 78, has gained strength in many surveys since having a heart attack last month. He has won the endorsement of three women of color in Congress, including Rep. Alexandria Ocasio-Cortez of New York, with whom he recently campaigned across Iowa as he firmed up support with progressive voters.

However, no candidate may have gained more from Warren’s struggles than Buttigieg, who was one of her most aggressive critics in the last debate.

Already a favorite of many in the Democratic donor class, whose support has allowed him to finance a well-funded advertising campaign, Buttigieg, 37, has gained support with a sharp pivot toward the political center.

Biden has absorbed blow after blow from his rivals and created a few challenges of his own, but it has yet to hurt him with his mostly older and nonwhite supporters.