Democratic presidential candidates, Sen. Bernie Sanders, I-Vt., left, and former Vice President Joe Biden, right, participate in a Democratic presidential primary debate at the Gaillard Center, Tuesday, Feb. 25, 2020, in Charleston, S.C., co-hosted by CBS News and the Congressional Black Caucus Institute. Credit: Patrick Semansky | AP

AUGUSTA, Maine — Former Vice President Joe Biden was the surprise winner of Maine’s Democratic presidential primary last week, one of several victories that have positioned him as the frontrunner heading into primaries in half a dozen states on Tuesday.

Biden won Maine by a slim margin, beating Vermont Sen. Bernie Sanders by a margin of just 1.5 percentage points. While the two continue to compete for their party’s nomination, Maine, which went mostly overlooked by candidates in the lead-up to the primary, looks to be in an unusual spot as a presidential bellwether.

Maine was supposed to be a friendly environment for Sanders. His narrow loss here suggests that other states he won in 2016 might not come as easily this time around. In 2016, Democratic caucus-goers in Maine overwhelmingly supported Sanders, giving him two-thirds of the state’s delegates. A few days later, the Vermont senator pulled off a surprising win in Michigan, signaling his staying power in the party’s primary that year.

But Sanders now faces an uncertain race in Michigan on Tuesday, with the state once seen as close to a sure win for him now more likely to go to Biden, according to the most recent polls. Three other states Sanders won in 2016 — Idaho, North Dakota and Washington — are also set to hold contests on Tuesday, and the Vermont senator no longer seems assured a victory in any of them in the race for the nomination to face President Donald Trump in November.

Sanders, who has been able to work with more money than Biden in the election cycle thanks to consistent fundraising from small-dollar donors, has had more sustained organization than the former vice president and has spent more on ads in several of the upcoming primary states.

The results in Maine last week show the limits of those traditional forms of campaigning. Biden won in Maine despite few endorsements, without running any ads in the state and without making a campaign stop or sending even one surrogate in his place.

Biden’s victory in Maine last week stemmed from a unique turn of events in the days leading up to the primary. Sanders looked like the clear favorite in Maine prior to the primary, leading in a Colby College poll in mid-February with 25 percent support, followed by former Mayor Pete Buttigieg of South Bend, Indiana, with 16 percent, former New York City Mayor Mike Bloomberg with 14 percent and Biden with 12 percent.

The former vice president likely picked up significant ground, however, when billionaire activist Tom Steyer, Minnesota Sen. Amy Klobuchar and Buttigieg dropped out of the race, with the latter two endorsing Biden. In an interview with Meet the Press on Sunday, Sanders cited the dropouts as a reason for his loss in Maine.

“If they had not withdrawn from the race before Super Tuesday, which was kind of a surprise to a lot of people, I suspect we would have won in Minnesota, we would have won in Maine, we would have won in Massachusetts,” he said.

Late-deciding voters and last minute dropouts assuredly helped Biden pick up votes. Several former Klobuchar and Buttigieg endorsers in Maine backed Biden just before the primary. Shayna Coombs of Bangor said she had planned on voting for Buttigieg, but when the former mayor dropped out a few days before the election, she decided to support Biden instead.

“I feel like he’s our best choice for beating Trump,” Coombs said.

BDN writer Charles Eichacker contributed to this report.