Joe Biden has won Minnesota, edging out a narrow victory against Bernie Sanders in the state as both Democratic candidates compete on Super Tuesday for the party's presidential nomination.

After dropping out of the race, the state's senator Amy Klobuchar backed the former vice president on Monday.

"We won Minnesota because of Amy Klobuchar", the former vice president told supporters in Los Angeles.

The state holds 75 delegates.

According to exit polls, nine out of 10 primary voters in the state are white, and a majority believed they needed to select a candidate they best believe can defeat Donald Trump in November, which they ranked as more important than electing a candidate for their policies. The state has an open primary that's not dependent on voters' party registrations.

Early projections show the former vice president and Mr Sanders neck in neck in the county that houses Minneapolis, represented by star Congresswoman Ilhan Omar, who backed the Vermont Senator.

He won the state in a decisive victory in 2016, beating Hillary Clinton by more than 20 points in a 61.6 per cent to 36.4 per cent vote.

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Minnesota voters reportedly overwhelmed caucus sites in 2016, prompting the state legislature passed legislation to turn the contest into a primary.

Though the state is frequently a Democratic stronghold, it became somewhat a battleground in the general election in 2016, when Ms Clinton beat the president by less than two points.