Sanders' victory in Maine, which was announced during CNN's Democratic debate in Flint, Michigan, is his third of the weekend after winning Kansas and Nebraska.
Sanders, 74, beat former secretary of state Clinton with 64.3 per cent of the vote compared to her 35.5 per cent across precincts in an expected win for the self-styled Democratic Socialist candidate who hails from nearby Vermont state and has strong support in the region.
"I thank the people of Maine for their strong support," the Sanders campaign said in a statement. But 68-year-old Clinton remains far ahead in the total number of delegates needed to secure the Democratic nomination for which a candidate should win at least 2,383 delegates.
For Rubio, 44, it's his second win in a campaign that has been dominated by real estate tycoon Trump and Texas Senator Ted Cruz. With 100 per cent of the votes counted on the island, Rubio topped the 50 per cent threshold required to win all 23 of Puerto Rico's delegates.
The dominant issue facing the island territory - which has no electoral votes in the US general election - is a public debt crisis, with the government owing USD 73 billion.

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It was the second outright win for Rubio, who also won Minnesota on Tuesday, but Trump, 69, has kept a firm grip on his lead in the Republican race, and called for Rubio to end his presidential bid.

Cruz has emerged as Trump's main rival in the weekend's primaries with mixed results. To secure the Republican Party nomination, a candidate has to win at least 1,237 delegates.


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