The Democrats used some of their harshest languages yet against Trump in their first matchup since the launch of a congressional impeachment inquiry into Trump's efforts to pressure Ukraine to investigate leading rival Joe Biden.
The unity gave way to some sharp clashes later in the debate over healthcare and a wealth tax. Democrats also found a new target, U.S. Senator Elizabeth Warren, who has surged into a virtual tie with Biden in many Democratic opinion polls.
Warren said she called for Trump's impeachment after former Special Counsel Robert Mueller's findings on Russian interference in the 2016 U.S. election.
Congress did not move to impeach, she said, "and look what happened: Donald Trump broke the law again."
Billionaire environmentalist Tom Steyer, who has poured millions into an impeach-Trump fund since 2017, tipped his hat to his rivals in his first debate.
"Every candidate here is more decent and coherent and patriotic than that criminal in the White House," said Steyer, the last candidate to jump into the race.
Pete Buttigieg, the South Bend, Indiana, mayor, turned on Senator Warren after she answered a question about whether taxes would go up under the Medicare for all government healthcare proposal by saying "costs" would go up.
That was "a yes or no question that didn't get a yes or no answer," Buttigieg said.
Sanders chided Warren for not being clear on whether Medicare for all plans, based on the government-run Medicare program for Americans 65 and older, would raise taxes. He said a majority of people would save money on their healthcare costs, but "I do think it's appropriate to acknowledge that taxes will go up."
Amy Klobuchar, a U.S. senator from Minnesota, piled on, telling Warren, "You are making Republican talking points" by proposing a plan that would eliminate private insurance. She added, "The difference between a plan and a pipe dream is something that you can get done.”
Businessman Andrew Yang said taxing wealth, as opposed to income, was a bad policy, attacking a position supported by several Democrats, including Warren and Sanders.
"We should not be looking to other countries' mistakes," said Yang. "Instead we should look at what Germany, France, Denmark, and Sweden still have, which is a value-added tax and we give the American people a tiny slice of every Amazon sale, every Google search, every robot truck mile, every Facebook ad, we can generate hundreds of billions of dollars and then put it into our hands because we know best how to use it."
Warren explained her plan, which she said would give young Americans better economic opportunities - taxing 2 percent of people's net worth above US$50 million and 3 percent over US$1 billion.
"My question is not why do Bernie and I support a wealth tax," Warren said. "It's why ... does everyone else on this stage think it is more important to protect billionaires than it is to invest in an entire generation?"
In an exchange on gun control, Buttigieg and former U.S. Representative Beto O'Rourke of Texas renewed a long-running rivalry.
The two have a history of disagreement on the subject. Buttigieg does not support O'Rourke's proposal to force people to sell some of their assault weapons and handguns to the government.
During the debate, O'Rourke described gun violence as a crisis and said politicians should take their lead from activists who are pushing for mandatory gun buybacks.
"Let's follow their inspiration and lead - and not be limited by the polls and the consultants and the focus groups," O'Rourke told Buttigieg.
"The problem isn't the polls; the problem is the policy," Buttigieg shot back. "And I don't need lessons from you on courage, political or personal."
Military veterans Buttigieg and U.S. Representative Tulsi Gabbard tussled over Trump's abrupt decision to pull U.S. troops out of northern Syria, which opened the door for Turkey to cross the border and attack the Kurds, a longtime U.S. ally.
Gabbard, an Iraq war veteran, and major in the U.S. National Guard said the assault on the Kurds was part of a long-failed U.S. policy of seeking "regime change" in the Middle East.
"Donald Trump has the blood of the Kurds on his hands, but so do many of the politicians in our country from both parties who have supported this ongoing regime-change war in Syria that started in 2011, along with many in the mainstream media," she said.
Buttigieg, who served in Afghanistan as part of the Naval Reserves, challenged Gabbard, saying she was pointing the finger in the wrong direction.
"The slaughter going on in Syria is not a consequence of American presence, it's a consequence of a withdrawal and a betrayal by this president of American allies and American values," Buttigieg said.
The three candidates leading in Democratic opinion polls - Biden, Warren, and Sanders, who are all in their 70s - were asked about their health.
Sanders, 78, recently suffered a heart attack that prompted a break from the campaign trail. He fielded the first question, about how he would reassure voters he can handle the stress of the presidency.
"Let me invite you all to a major rally we're having in Queens, New York," Sanders said, noting that there would be a "special guest," shortly before his campaign confirmed the endorsement of freshman U.S. Representative Alexandria Ocasio-Cortez. "We are going to be mounting a vigorous campaign all over this country, that is how I think I can reassure the American people."
Biden, 76, said he knows he could handle the demands of the presidency because he knows what the job entails. "One of the reasons I'm running is because of my age and my experience," Biden said, promising to release his health records before the first nominating contest in early February.
Warren, 70, was asked to respond to a statistic that 40 percent of Democratic primary voters say they think a candidate under 70 is more likely to defeat 73-year-old Trump.
"I will outwork, out-organize, and outlast anyone, and that includes Donald Trump, (Vice President) Mike Pence, or whoever the Republicans get stuck with," Warren said.