The United States Democratic presidential primary is down to two major candidates, and it shows.

Former Vice President Joe Biden and Vermont Senator Bernie Sanders are spending their first weekend as their party's last top White House contenders sharpening their attacks against one another.

Each is trying to demonstrate that he is the best choice before six more states - Idaho, Michigan, Mississippi, Missouri, North Dakota and Washington - vote on Tuesday.

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It reflects the new contours of a race that once featured more than 20 Democrats. This state of play could endure for months as Biden and Sanders wage a protracted battle for the right to face President Donald Trump in November.

"You cannot defeat Trump with the same-old, same-old politics of yesteryear," Sanders told more than 7,000 supporters at a convention hall in downtown Detroit.

'From the bottom up'

At 78, Sanders is actually a year older than Biden. But the avowed democratic socialist who has served in Congress since 1991, argues that he has bucked the establishment of both parties with decades of unpopular stands that now give him the credibility to lead a political revolution "from the bottom up".

Sanders says it is part of a larger movement that can draw younger voters, minorities and working-class people to the polls even though they tend to vote in lower concentrations than many other Americans.

Strong support among Hispanics lifted Sanders to victories in Nevada and California, but Biden trounced him in South Carolina and throughout much of the Deep South that voted during last week's Super Tuesday. Biden especially ran up the score with the African Americans.

Sanders is looking for a strong finish in Washington. But he cancelled a trip to Mississippi to focus on Michigan, the largest prize on Tuesday.

He was holding a rally on Saturday in the heavily Arab American community of Dearborn, and had three more Michigan events scheduled this weekend. Biden was campaigning in Missouri and Mississippi.

Sanders has used his Michigan stops to hammer Biden's past support for the North American Free Trade Agreement, arguing that it moved high-paying US jobs to Mexico and China while devastating manufacturing in a state dominated by the car industry.

"Joe's been around for a while and I've been around for a while. How do we differ? What's our records? Who stood up when the going was tough?" Sanders said.

He has focused on Biden's years in the Senate, when Biden backed not only trade agreements and the US-led war in Iraq, but also a ban on using federal funds to pay for abortions. Biden announced this summer that he was reversing his position on that, but Sanders said that was not enough.

"I think we need a candidate that can be trusted on this issue. I am proud to tell you that I am 100 percent pro-choice," Sanders said.

Biden saw a surge of donor support after South Carolina and Super Tuesday, and his campaign announced that it was spending $12m on a six-state advertisements buy in places voting this Tuesday and the following week. It was his largest single advertising effort of the 2020 campaign.

He is using two television and digital advertisements, one promoting his relationship with former President Barack Obama, the other a new effort to counter a Sanders attack on Biden's past record on Social Security. It is a criticism Sanders has used for months, though he has not mentioned it as frequently while campaigning in Michigan.

"Biden will increase Social Security benefits and protect it for generations to come," a narrator intones in one of the ads, before turning the matter back on Sanders. "Negative ads will only help Donald Trump. It's time we bring our party together."