Charleston, South Carolina (CNN) Vermont Sen. Bernie Sanders is the Democratic front-runner -- and now, he's feeling the heat.

Sanders faced pointed questions from CNN town hall attendees in Charleston on Monday night, as well as attacks from former South Bend, Indiana, Mayor Pete Buttigieg and businessman Tom Steyer

Buttigieg's and Steyer's eagerness to attack Sanders showed that the front-runner is now the candidate everyone else in the Democratic race is targeting, with South Carolina's primary coming Saturday and Super Tuesday looming next week. It also foreshadowed a heated debate on Tuesday night.

Here are three takeaways from Monday night's CNN town halls:

The front-runner takes the hits

On the night before he takes the debate stage in South Carolina as the unquestioned Democratic primary front-runner, Sanders faced a lot of the same questions his rivals are expected to weaponize against him onstage.

Sanders stood by his comments praising the literacy program implemented by Cuba's late leader Fidel Castro , even after they drew bipartisan criticism . He denied reporting that he considered a primary challenge against President Barack Obama -- something former Vice President Joe Biden has used to attack to criticize the senator in recent days. And, faced with continued questions over how he would pay for his massive government programs, Sanders pulled out papers he said would explain how his plans were all paid for.

His answers, though, were unlikely to satisfy the other candidates or their supporters.

Sanders suggested the members of Congress who criticized his praise of Castro's literacy program are "coincidentally" backing other candidates. But the two most vocal critics, Florida Reps. Debbie Mucarsel-Powell and Donna Shalala, have not endorsed anyone.

And the documents Sanders brought with him to the town hall, which were emailed out by his campaign, look a lot like what he's already made public.

Sanders has consistently denied ever weighing a run against Obama -- saying a recent report in The Atlantic that he did was wrong. But even the disputed piece of political history could become a flashpoint during the debate.

There was also the question of the aggressive online behavior of some of his supporters.

"I'm not going to tell you we don't have some jerks out there," he said.

But, Sanders added: "I do want to say to those folks: We do not want your support if you think that what our campaign is about is making ugly attacks on other candidates. We don't want you."

Buttigieg aims directly at Sanders

CNN Democratic Presidential Town Hall with Former South Bend, Indiana, Mayor Pete Buttigieg moderated by CNN's Don Lemon College of Charleston Charleston, South Carolina February 2020

Buttigieg used his CNN town hall to signal how he is approaching the next phase of the Democratic primary: all Bernie Sanders, all the time.

Buttigieg started on Monday by contrasting himself with Sanders, telling the South Carolina voters that he believes "in calling people into the tent, not calling them names online."

The line is a subtle jab at some of the virulent comments Sanders supporters make online.

Then Buttigieg hit Sanders for the senator's laudatory comments about Castro, asking Democrats if they "want to be explaining why our nominee is encouraging people to look on the bright side of the Castro regime going into the election of our lives." And Buttigieg, when asked why he should be the one to take on Sanders, said, "I'm the best alternative ... because I'm the only one who's beat him this year, anywhere."

The focus on Sanders hints at two keys to the former mayor's campaign.

Buttigieg knows his campaign needs Sanders to slip in order to have any chance to win the nomination. The former mayor said as much during the last debate.

But Buttigieg's performance on Monday night also signaled how the former mayor will be entirely focused on Sanders at Tuesday night's debate.

Steyer swings at Sanders, too

Steyer had just walked onto the stage -- and hadn't even been asked a question yet -- when he lit into Sanders.

"I don't think a government takeover of major parts of the American economy is a good idea," Steyer said of Sanders' policy proposals. "I don't think it's good for working people, I don't think it's good for families."

He said he knows "unchecked capitalism has failed," but the answer, he argued, was to "break the corporate stranglehold" on the government.

Steyer later said it is "inappropriate" for Sanders to praise Castro's regime for its literacy efforts.

The businessman said he would never praise "unelected leaders of countries who completely control without any form of democracy, justice or equality."