In an email to supporters following the debate, Sanders struck an optimistic tone, looking forward to Democrats' Michigan nomination contest Tuesday.

Four minutes after caucuses closed, the Associated Press called the race for Sanders Sunday. As of midnight, the self-identified democratic socialist led with 64 percent over the former secretary of state's 36 percent support, with 91 percent of precincts reporting.

Augusta, ME - Maine's high-turnout caucuses delivered Bernie Sanders his fourth state primary win of the weekend Sunday, moments after he took the stage alongside national frontrunner Hillary Clinton for the seventh Democratic debate.

"While many in our country's political and financial establishment would love to see our campaign go away, tonight proved once again that we are well on our way towards winning this Democratic primary," he told supporters.

Sanders exited the weekend winning three of Democrats' four state primaries, and gaining three delegates more than Clinton between Kansas, Nebraska, Louisiana and Maine. Clinton still holds a wide national lead, with 1,130 delegates (including super delegates) to Sanders' 499, according to the AP.

Michigan and its fellow winner-take-all primaries ahead are must-wins for Sanders to close that gap. Polls, however, project Clinton will win the Mitten State.

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Turnout

Particularly after similarly high turnouts boosted him to victory in Nebraska and Kansas Saturday, it was little surprise that Maine's nomination contest favored Sanders Sunday. The independent senator from Vermont has fared well in the northeast, and generally plays well in states like Maine, where white voters are the dominant demographic.

Due to what the Maine Democratic Party called "extraordinary turnout," some voters lining up to caucus were asked instead to vote by paper ballot, to avoid overcrowding.

Maine Democratic Party chair Phil Bartlett tweeted late Sunday that more than 47,000 attended caucuses, an "unprecedented" number. The previous record, set in 2008, was roughly 44,000 caucus-goers, the Portland Press Herald reports.