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Democratic presidential hopefuls will pile on to the stage at Ohio's Otterbein University in a few hours for what will be the largest primary debate in US political history. A dozen Democrats qualified for the fourth debate under the rules set out by the Democratic National Committee. It will be co-hosted by CNN and The New York Times.

The candidates include billionaire Tom Steyer, who will be making his first appearance on a debate stage. Hawaii Rep. Tulsi Gabbard, who did not qualify for the September debate, will also return. Gabbard threatened to boycott Tuesday's debate over what she called a “rigging” of the election by Democratic Party leaders and corporate media, but reversed course Monday, saying she'd attend after all.

The debate will also feature Vermont Senator Bernie Sanders' return to the campaign trail after he was treated in hospital for a heart attack earlier this month.