Former Vice President Joe Biden and Vermont Senator Bernie Sanders are spending their first weekend as their party's last top White House contenders increasingly taking aim at one another.

Each wants to show he's the best choice before six more states - Idaho, Michigan, Mississippi, Missouri, North Dakota and Washington - vote on Tuesday.

It reflects the new contours of a race that once featured 20-plus Democrats. An increasingly bitter matchup could endure for months as Biden and Sanders compete for the right to face President Donald Trump in November.

"We have a two-person race," Sanders said Saturday in Dearborn, a Detroit suburb with one of the nation's largest Arab American populations.

"And all over this country, people are asking themselves which candidate can best defeat Trump. I have zero doubt in my mind that, together, we are the campaign that can beat Trump."

Campaigning in St Louis, Biden said he was the one to unite the party and the country, and he would do that by promoting an upbeat message.

"If you want a nominee who'll bring the party together, who will run on a positive progressive vision for the future, not turn this primary into a campaign of negative attacks - because that will only re-elect Donald Trump if we go that route - if you want that, join us," Biden said.

But Biden also gently knocked Sanders' weeks of suggestions that he is the candidate who can prompt record voter turnout in November and defeat Trump, saying that actually "we're the the campaign that's going to do that."

Sanders is clearer in drawing contrasts, arguing that no Democrat will win the presidency "with the same-old, same-old politics of yesteryear."

That's ironic given that the 78-year-old Sanders is actually a year older than Biden.

But the avowed democratic socialist, who has served in Congress since 1991, says he's bucked the establishment of both parties with decades with unpopular stands that now give him the credibility to lead a political revolution "from the bottom up."

Sanders is pledging to increase Democratic turnout by drawing 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 African Americans.

Some activists are disappointed that a once diverse field of women and minorities has dwindled to two white men in their late 70s.

Top advisers expect Sanders to finish strong in Washington. Still, he cancelled a trip to Mississippi to focus on Michigan, Tuesday's largest prize.

He made a stop in Chicago's Grant Park on Saturday afternoon, and declared that he has a different vision than Biden, "And the American people are going to hear about it." Sanders will spend the rest of the weekend in Michigan, while Biden is in Missouri and Mississippi.

Sanders said repeatedly that he and Biden are friends and that, if he's not the nominee, he will support Biden against Trump. But, he added, "In the remaining months, I intend to make it clear what my views are and what Joe Biden's are."

Sanders has used many of his Michigan events 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 auto industry.

He's 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 has since reversed his position on that, but Sanders said that wasn't enough.

Australian Associated Press