Democratic presidential rivals Joe Biden and Bernie Sanders are racing across the US midwest this weekend, with the progressive Sanders sharpening his attacks while the former vice-president warned against a campaign “bloodbath” as the 2020 nominating contest narrows.

Biden also warned, in an intensifying fight, against what he called “Bernie brothers” – understood to be a reference to diehard fans of Sanders more widely dubbed “Bernie bros” and known for aggressive online attacks on supporters of Democratic rivals and an unwillingness to join up with them if Sanders loses the nomination.

“What we can’t let happen is let this primary become a negative bloodbath,” Biden told more than 100 donors gathered at a private residence in Bethesda, Maryland. “I know I’m going to get a lot of suggestions on how to respond to what I suspect will be an increasingly negative campaign that the Bernie brothers will run. But we can’t tear this party apart and re-elect Trump,” Politico reported late Friday.

Before dropping out earlier this week, Elizabeth Warren had called out Sanders for what she called nastiness and bullying of her supporters by some of his supporters.

Biden and Sanders are now preparing for a showdown in the key midwest primary contests in Michigan and Missouri on Tuesday.

Sanders, 78, who until recently was the frontrunner in the party’s race to face Republican White House incumbent Donald Trump in November, is now trailing in his total of pledged delegates, the senior party members who eventually anoint the nominee.

He needs to regain momentum after Biden, 77, received a rush of endorsements from party establishment figures following his strong comeback on Super Tuesday, 3 March.

Sanders ramped up his attacks on Biden on Friday, speaking in Arizona and then Michigan after canceling a rally in Mississippi, effectively conceding the state after Biden swept the majority of the southern black vote earlier in the week.

And the moderate Biden told a crowd at a fundraiser in Maryland, he was worried the battle could turn ugly.

“We have to keep our eyes on the ball,” Biden said.

Sanders, despite saying he and Biden were friends, asserting either could beat Trump and that both would be certain to rally behind the eventual nominee, assailed Biden’s record on issues such as trade, pensions, gay rights, abortion rights and the climate crisis, both on the campaign trail and via Twitter.

In 2007, NBC reported that as president, Joe Biden’s Social Security plans "would include discussing options such as upping the retirement age.” 7/8 https://t.co/jRKP2d7byB — Bernie Sanders (@BernieSanders) March 7, 2020

The Democratic race became a two-man contest between Biden and Sanders after contenders Elizabeth Warren, Mike Bloomberg, Amy Klobuchar, Pete Buttigieg and Tom Steyer all dropped out in the last week.

Rejoining the Paris Agreement isn't enough. Scientists say we must act immediately to end our dependence on fossil fuels and avoid climate catastrophe.



Our Green New Deal, which will create 20 million jobs, is the only plan in this race bold enough to address this crisis. https://t.co/njXDJdH4G3 — Bernie Sanders (@BernieSanders) March 7, 2020

Tulsi Gabbard, a Hawaii congresswoman, is the only other Democratic candidate still in the nominating contest, but has virtually no chance of winning.

She won’t currently meet party rules to qualify for the next Democratic debate on 15 March in Phoenix, Arizona, where now only Biden and Sanders will compete on stage. Late Friday Gabbard demanded support to help her get into the debate, encouraged by another 2020 dropout, Marianne Williamson.

Idaho, Mississippi, Missouri, Washington state, North Dakota and Michigan vote on March 10 in what is dubbed Mini Super Tuesday.

The former North Dakota senator Heidi Heitkamp has endorsed Biden.

I'm endorsing @JoeBiden because he understands the America that showers after work. He knows personally what happens when families can't pay the grocery bill. He'll keep his commitments to our farmers & ranchers. And he'll restore empathy for others to the White House. #JoeMentum pic.twitter.com/VxanvhZBbt — Heidi Heitkamp (@HeidiHeitkamp) March 6, 2020

A big win for Biden in delegate-rich Michigan would deliver a major blow to Sanders’ hopes of becoming the nominee.

Biden’s campaign said it had raised about $22m in five days.

In Missouri Jay Nixon, the state’s former Democratic governor, swung his support behind Biden, leading a well-choreographed army of Missouri officials – 68 in all – to come out in support, and Gretchen Whitmer, the popular Michigan governor, has endorsed Biden.

Biden was due to campaign on Saturday in St Louis and Kansas City, Missouri.

Bernie Sanders speaks at a campaign rally at Salina Intermediate School on 7 March 2020 in Dearborn, Michigan. Photograph: Bill Pugliano/Getty Images

Sanders begins a weekend of campaigning in the crucial battleground state of Michigan, which offers 125 of the 1,991 delegates needed to win the nomination.

Sanders, who unexpectedly won the Michigan primary in 2016 when he ran unsuccessfully for the nomination against Hillary Clinton, held a rally on Saturday afternoon in the Detroit suburb of Dearborn.

He lambasted Biden for voting for the Iraq war and for trade deals he said cost millions of American jobs, including in Michigan.

“Joe Biden voted for those agreements…[they] turned out to be an absolute disaster,” Sanders said, referring to deals such as the 1994 North American Free Trade Agreement (Nafta).

Sanders also decried Biden’s acceptance of campaign contributions from billionaires.

“People understand that if you’re taking money from billionaires, you’re not going to be there standing up for the working class and the middle class of this country,” Sanders said.

He plans a town hall debate focused on racial and economic justice in Flint, which is still grappling with the effects of a crisis involving lead leaking into the public water supply, and on Sunday heads to Grand Rapids and Ann Arbor.

Meanwhile Biden reportedly launched his biggest advertising blitz yet, $12m on ads in key states voting on the next two Tuesdays, chiefly Michigan, Missouri, Mississippi, Florida, Ohio and Illinois.

At a rally in St Louis on Saturday afternoon, Biden pledged: “We’re going to unite this party and unite this country.”

He thanked former Democratic rivals who have very recently dropped out, and also unexpectedly thanked supporters of former 2020 rival Kamala Harris, fueling speculation that the California senator might be about to endorse him.