Hillary Clinton Hillary Diane Rodham ClintonHillicon Valley: FBI chief says Russia is trying to interfere in election to undermine Biden | Treasury Dept. sanctions Iranian government-backed hackers The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters FBI chief says Russia is trying to interfere in election to undermine Biden MORE and Bernie Sanders Bernie SandersMcConnell accuses Democrats of sowing division by 'downplaying progress' on election security The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters Why Democrats must confront extreme left wing incitement to violence MORE are poised for a pivotal debate on Sunday with just two weeks to go before the Feb. 1 Iowa caucuses.

Sanders is surging in polls, and a sweep of the first two contests in Iowa and New Hampshire appears within his grasp.

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Anxiety appears to be building in the Clinton campaign, which has seen the race tighten.

Clinton’s performance at the first Democratic debate in October was a cornerstone of Clinton’s strong fall, which underlined her status as the clear frontrunner.

Coupled with a grueling but successful day of testimony before the House Benghazi panel in which Republicans failed to land a solid blow, the solid debate performance quieted doubts about the former secretary of state’s campaign.

Since then, there have been two more debates — neither of which was memorable. One came the day after the terrorist attack in Paris, while the third was held on the Saturday before Christmas.

Sunday’s affair is also out of the limelight: It will take place in the middle of a three-day holiday weekend for many voters.

Yet it represents a clear second shot of sorts for Sanders, who kept the gloves on in his first on-stage encounter with Clinton but not has a chance to reengage under a media spotlight.

Team Sanders is taking a cautious approach heading into the fight.

“We don’t feel like we have to go in there and heat it up,” Sanders advisor Tad Devine told The Hill.

“But if she starts to say he’s going to dismantle health care in America or some of the stuff they’ve been peddling out there on the campaign trail, he’s not going to let that slide. He’s going to engage on it and he’s going to be forceful.”

In the past week, Clinton has characterized Sanders as standing against her and President Obama on gun control, and attacked Sanders’ call for single-payer health care as an idea that would “end all the kinds of health care we know.”

Team Clinton also accused Sanders of launching a negative television ad, a day after the campaign put former first daughter Chelsea Clinton on the stump to go after Sanders on healthcare.

The decision to use Chelsea Clinton for an attack on Sanders, and the decision to go after Sanders over his support for a single-payer health system have both raised eyebrows.

Chelsea Clinton’s strength is not necessarily as an attack surrogate, some Democrats said.

Others suggested a Democratic primary audience would generally back a single-payer health system, despite Clinton’s arguments about its cost.

Democratic strategist Peter Fenn said Clinton’s approach is to portray Sanders’s policies as too “pie in the sky,” an argument that also could be used to attack the self-professed Democratic socialist’s electability.

“She wants to bring some of this stuff down to Earth,” Fenn said.

Both candidates need to appeal to liberal voters, who are likely to carry the day in both Iowa and New Hampshire.

Tobe Berkovitz, a professor of communications at Boston University who specializes in political communication, said for Clinton and Sanders it will be “how far and how fast can you run to the left. You're not looking to South Carolina yet.”

While momentum appears to be on Sanders’s side, he’s still an underdog.

In Iowa, a win by Clinton could all but end the campaign. Many believe Sanders needs to win both states to alter a race that will then head south, where minority voters could be a big factor for Clinton.

Clinton is also looking for a good performance on Sunday.

“She needs to recreate the kind of big impact she had in the first debate, where she demonstrated that she’s an adroit explainer of policy,” said David Birdsell, a Baruch University professor and debate expert.

Berkovitz argued that Clinton has “more to lose.”

“Three months ago, who would have thought that Bernie Sanders would be a player in this election?” she said.

One Clinton donor who has helped raise money for the campaign acknowledged that the debate would almost assuredly be a fundraising boon to Sanders, who has seen “absurd amounts of money in recent days.”

“She has to be worried about his momentum,” the donor said. “If he wins Iowa and then he goes on to win New Hampshire, suddenly he has a lot of momentum and suddenly other people across the country start taking a look.”