In a heated debate in New Hampshire just four days before the state's pivotal primary, their Democratic rivals questioned whether Sanders' democratic socialist views and Buttigieg's relative lack of experience and lack of support with African-Americans and Latinos made them too much of a risk for the Nov. 3 election.
Sanders, 78, a U.S. senator and leader of the party's progressive wing, and Buttigieg, the 38-year-old former mayor of South Bend, Indiana, finished in a virtual dead heat in Iowa's first-in-the-nation caucuses earlier this week.
Former Vice President Joe Biden, the one-time national front-runner who finished a disappointing fourth in Iowa, delivered his most aggressive performance yet, a sign of possible desperation as he looks to rebound.
Biden said Trump would have an easy time ripping into Sanders in a general election campaign, and Sanders would drag down other Democratic candidates in federal and state races.
"Bernie has labeled himself, not me, a democratic socialist. I think that’s the label that the president is going to lay on everyone running with Bernie if he is the nominee," Biden said.
U.S. Senator Amy Klobuchar, who finished a distant fifth in Iowa, said Sanders would not attract the kind of centrist voters Democrats need to win.
"Donald Trump’s worst nightmare is a candidate who will bring people in from the middle," Klobuchar said at the eighth Democratic debate. "I think we need someone to head up this ticket that actually brings people with her instead of shutting them out."
Sanders, who has called for a political revolution that will attract new voters, said "the way to beat Trump is by having the largest voter turnout in the history of this country." He said he could appeal to working class voters who have given up on the political process.
Biden and Klobuchar also questioned whether Buttigieg had enough experience to face off with Trump.
Buttigieg, who served two terms as mayor of South Bend, a city with a population of 100,000, said the Washington insider experience of some of his rivals was no longer what was needed, and it was time to "turn the page" on the old Washington politics.
"It is easy to go after Washington, because that’s a popular thing to do," Klobuchar said, telling Buttigieg that "it’s popular to say and makes you look like a cool newcomer."
'I WILL PROBABLY TAKE A HIT'
Historically, candidates who win the Iowa caucuses see a boost in New Hampshire, and two opinion polls released this week showed Buttigieg within striking distance of Sanders, who has consistently been atop the field in the state.
Biden did not seem confident about how he would do.
"This is a long race. I took a hit in Iowa, and I’ll probably take a hit here," he said.
With controversy surrounding the Iowa caucuses - technical problems and inconsistencies delayed the results for days - New Hampshire's primary on Tuesday takes on added importance.
Billionaire activist Tom Steyer, who has focused heavily on upcoming primaries in the more diverse states of Nevada and South Carolina, launched a lengthy discussion of race and noted the party's broad diversity - and the mostly white candidates on the stage aside from Andrew Yang, an Asian-American.
He took a swipe at Buttigieg, saying he has not been able to show much appeal to the black and Latino voters who are crucial to a winning Democratic campaign.
"Unless you can appeal to the diverse parts of the Democratic Party, including specifically the black community, including specifically Latinos, if you can’t do that, then we can’t beat Donald Trump in November," Steyer said.
Buttigieg came under fire for his record on race in South Bend. When asked about an increase in arrests of blacks on marijuana-related charges, he said as mayor he targeted cases "when there was gun violence and gang violence."
Asked if Buttigieg's answer was sufficient, U.S. Senator Elizabeth Warren said: "No. You have to own up to the facts."
Democratic rivals also had moments of unity - when the subject of Donald Trump came up.
Biden criticized Trump's firing on Friday of Army Lieutenant Colonel Alexander Vindman, the top Ukraine expert at the White House's National Security Council, two days after Trump was acquitted in his Senate impeachment trial.
Vindman served as a star witness during the House of Representatives' impeachment proceedings.
Biden said Trump "should be pinning a medal on Vindman and not Rush Limbaugh," a reference to the president’s decision to give the Presidential Medal of Freedom to the conservative radio host earlier this week.
"Stand up and clap for Vindman," Biden said, encouraging a standing ovation in the debate hall.
Klobuchar praised Republican U.S. Senator Mitt Romney and Democratic U.S. Senator Doug Jones of conservative Alabama for having the "courage" to vote to convict.
Notably absent from the debate was Michael Bloomberg, a billionaire former New York mayor, who is not competing in New Hampshire but has been assembling a formidable operation in later voting states.
He has been funding his own campaign and not taking donations, so he failed to meet the donations criteria for the debate. The Democratic National Committee dropped the donor requirement for the next debate in Nevada, however, possibly opening the door for Bloomberg.