Just a few months ago, former Secretary of State Hillary Clinton enjoyed a huge lead over Sanders in both Iowa and New Hampshire, but things have definitively changed.
The latest polls show Sanders leading Clinton by 22 points in New Hampshire and by 10 points in Iowa.
"We have proceeded more quickly than I thought we would. We had perceived at the beginning there would be a more gradual ramping up ... Now this gives us the opportunity to campaign in some of the states further down the calendar so that we can become known there as well," says Jeff Weaver, Sanders' campaign manager.
At this point in the campaign, Weaver says he's not attaching too much importance to the poll numbers.
"We are not taking anything for granted. We're going to fight for every vote but people should be prepared that polls are going to swing up and down throughout this campaign," Weaver says.
Retired Middlebury College political science professor Eric Davis says Sanders' new strength in the polls reflects an enormous growth in his paid campaign staff in both Iowa and New Hampshire.
"In terms of mobilizing voters and getting his message out he certainly has the resources in those two states and I think now we realistically need to look at not just him doing better than expectations in both Iowa and New Hampshire but actually winning both of those states," he says.
Davis says Sanders now faces some unexpected challenges as the front runner in the first two states.
"Perhaps one of the things Sanders has to be aware of now is the expectations become so high that if he ends up finishing a close second rather than winning in either one of those states, it's seen as a setback for him rather than an accomplishment," he says.
And Davis says Clinton's dramatic drop in these polls could encourage another Democratic candidate to enter the race after the party's first debate in mid-October.
"If one of the perceptions of voters after this debate is they may have reservations about Clinton but aren't yet willing to commit to Sanders, that could encourage Biden or somebody else to get into the race late," Davis says.
Vice President Joe Biden, who is seriously considering becoming a candidate, received 10 percent of the vote in both Iowa and New Hampshire polls.