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As Pete Buttigieg and Amy Klobuchar suspend their campaigns to endorse Joe Biden, pressure mounts on the former Vice-President to emerge as the clear challenger to Vermont Senator Bernie Sanders.

The democratic socialist has an early lead and big endorsements from left-wing Democrats and celebrities.

Meanwhile, Elizabeth Warren, Tulsi Gabbard and new entrant Michael Bloomberg are battling to turn the race into a three-way competition.

It was Super Tuesday in 2016 that saw Donald Trump and Hillary Clinton established as the clear contenders for the presidency.

California, Texas, Virginia, Massachusetts, Tennessee, Alabama, Arkansas, Oklahoma, Minnesota, Vermont, Colorado, Utah, North Carolina and Maine - as well as American Samoa, will all be voting to choose their preferred presidential candidate.

David Grossman reports and Emily Maitlis speaks to David Frum and Krystal Ball