Former US Vice President Joe Biden speaks during a rally in Philadelphia May 18, 2019. (Xinhua/Liu Jie)

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.

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 favor.

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. "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 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.

Sanders, speaking to an equally enthusiastic crowd in Chicago, underlined his differences with Biden, without directly attacking him.

"Joe Biden is a friend," he said. "I have known him for many years. But we have records, we have a different vision. The American people will hear about it."

With the primary now "down to two people," the progressive Vermont senator said, "it is important for the American people to understand the differences between us - in terms of our record, in terms of our vision for the future."

Sanders is a democratic socialist who wears his uncompromising positions, which are government-run health care for all, higher taxes on the wealthy and free university tuition, proudly on his sleeve.

Biden is a centrist who prides himself on his ability to work with Republicans. He is more middle-of-the-road on health care, where he favors expanding existing insurance programs, and less punitive additional taxes on the wealthy. 

At a later rally in Flint, Michigan, Sanders focused on the "real differences" he has with Biden.

These include what Sanders says is Biden's support for bad trade deals that cost the US "over 4 million good paying jobs."

AFP