When he launched his campaign at the end of May, many members of the national news media considered Sanders to be a fringe candidate. Now he's thought of as the main challenger to former Secretary of State Hillary Clinton.
Retired Middlebury College political science professor Eric Davis says this type of personal campaigning works well in the early states of Iowa and New Hampshire because voters expect to meet candidates.
But at the beginning of March, Davis says, the Democratic race shifts to much larger states.
"That's when things are going to become challenging… these bigger states depend more on the candidate getting his message out or her message out through television advertising and ‘wholesale politics,’ so to speak, rather than ‘retail politics,'” said Davis.
Davis thinks these larger states pose another critical challenge to the Sanders campaign.
“Polls indicate that he's having difficulty connecting with African-American voters, with Hispanic voters, with other voters of color within the Democratic primary and caucus election. And that's going to be a challenge for him,” Davis said.
Megan Remmel is a political science professor at Norwich University in Northfield, Vermont. She says Sanders will have to expand his base of supporters to be successful after Iowa and New Hampshire.
“He's going to need to up his game in states like Nevada and South Carolina where's there's a much bigger minority population. That will really tell the tale of whether or not he can appeal to a much wider part of the electorate and a much more diverse section of the Democratic Party,” Remmel said.
If Sanders’ popularity continues to surge, Remmel expects that national polls will include him in one-on-one matchups with various Republican candidates. She thinks this poses a risk for Sanders.
“Say he doesn't do all that well against somebody like a Jeb Bush. That could scare people back to the Clinton camp because now they're afraid they're going to lose the general election,” Remmel said.
University of Vermont political science professor Garrison Nelson says Sanders' economic message is playing well with people who feel the political system doesn't work for them. Nelson says the challenge for Sanders is getting these people to vote.
"I think that's what he has to count upon... they're angry but they feel sufficiently empowered to vote for him, believing that he can make a difference,” Nelson said.
Former Middlebury professor Davis says Sanders could also face another major challenge if he continues to rise in the polls against Hillary Clinton.
Davis thinks other well-known Democratic candidates might jump into the race if they believe that Clinton has lost her appeal to voters.