Hillary ClintonHillary Rodham ClintonDNI official challenges reports of low morale in intelligence community Why the GOP cannot sweep its Milo scandal under the rug Dems worry too much about upsetting others. That needs to stop. MORE’s campaign is “worried” about Bernie SandersBernie SandersSanders: 'If you don't have the guts to face your constituents,' you shouldn't be in Congress President Trump, immigrants are not 'bad dudes' Why the GOP cannot sweep its Milo scandal under the rug MORE, whom a top Clinton aide described as a “serious force” in the 2016 battle.

“We are worried about him, sure. He will be a serious force for the campaign, and I don’t think that will diminish,” Clinton Communications Director Jennifer Palmieri said Monday in an interview with MSNBC's "Morning Joe."

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“It's to be expected that Sanders would do well in a Democratic primary, and he’s going to do well in Iowa in the Democratic caucus.”

Sanders, an independent senator from Vermont, has emerged as Clinton's main foil in the Democratic primary.

While he's still more than 40 percentage points behind Clinton in virtually all national polls, he's greatly improved his stock in the early primary states. 

A new Quinnipiac University poll released last week found he doubled his share of Democratic supporters in Iowa in just seven weeks. Some polls in New Hampshire show Sanders less than 10 points behind Clinton.

Palmieri said Sanders's rise won't prompt a shift toward negative campaigning and that Sanders's strong crowds only underscore the differences in the campaigning tactics between the two campaigns. 

“We don’t need to attack each other. He'll run his campaign, we'll run ours. The imperatives for us are different. We think what works for her, particularly in Iowa, is doing a lot of small events, staying a long time, being one of the last people, if not the last person, to leave the room. That works better for us right now,” she said.

“It's going to be a slog, but I feel like she will win.”