Supporters of Joe Biden (Photo: AFP)

Joe Biden has won the Democratic primary in Alabama, US networks projected, racking up his third win in the crucial Super Tuesday contests to pick a challenger to President Donald Trump.

The Deep South state, which has 52 delegates, was easily won by Hillary Clinton in 2016 and centrist Biden was set to replicate that result, handily beating out his main rival, the leftist Senator Bernie Sanders. 

Biden was earlier projected winner in North Carolina and Virginia, while Sanders was projected to pick up an easy win in his home state Vermont.

US networks meanwhile called the US territory of American Samoa -- which has six delegates -- for billionaire Michael Bloomberg, giving the former New York mayor his first win of the race.

Though Biden now has three wins to Sanders' one, several of the early states were known to favor the former vice president.

Self-professed democratic socialist Sanders is the strong favorite in California, the biggest prize of the night with 415 delegates, where polls won't close until 11:00 pm EST (0400 GMT Wednesday.)

He is also hoping for a strong showing in Texas, where 228 delegates are up for grabs.

A total of 14 states voted on Tuesday, with 1,357 delegates at stake -- a third of the nationwide total. Biden needs a good performance to prevent Sanders from taking a potentially insurmountable lead into the party's nominating convention in July.