Buttons are seen for sale outside Democratic presidential hopeful Bernie Sanders campaign rally at Salina Intermediate School in Dearborn, Michigan, on Saturday. Jeff Kowalsky/AFP

Joe Biden and Bernie Sanders on Saturday began what amounts to a fresh campaign for the Democratic nomination for president, as the septuagenarians prepare to go head-to-head at the polls for the first time since the field narrowed to two credible candidates.

Democratic presidential candidate Joe Biden speaks during an event in Los Angeles. File/AFP

Biden, the 77-year-old former vice president, spoke to a large crowd of supporters in Missouri, one of six states that will hold Democratic primaries on Tuesday, one week after the "Super Tuesday" elections brought about a dramatic reversal of fortunes in his favour.

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Standing on an outdoor stage on a sunny day in St Louis, at times wearing his signature aviator sunglasses, the politically moderate Biden savored his spectacular revival in the race for the White House.

Bernie Sanders arrives during a rally at the Champlain Valley Expo in Essex Junction, Vermont. File/AFP

"What a difference a day makes," he exulted. "This time last week I was in South Carolina and the press and the pundits had declared Biden's campaign dead."

"But South Carolina had something to say about that, and then came Super Tuesday. And today there are 11 victories behind us and we're leading both in delegates and national votes."

With the monumental Gateway Arch — a symbolic entryway to the American West — in the background, the former vice president mentioned Sanders only indirectly.

Biden, having gained key backing from erstwhile rivals Pete Buttigieg, Amy Klobuchar and Michael Bloomberg, said he was best positioned to "unite this party," promising not to turn "this primary into a campaign of negative attacks."

"That will only re-elect Donald Trump if we go that route," he said.

Agence France-Presse