Cory Booker, the New Jersey senator struggling to be the only black candidate on the Democratic debate stage this month, has warned that the party could hand re-election to Donald Trump unless it sends a more positive message to African American voters.

Booker has just four days left to meet stringent criteria set by the party for the next televised primary debate, in Los Angeles on 19 December. Should he fail to make the cut, the participants will be exclusively white, with more billionaires on stage than black people.

In an interview with the Guardian, Booker said he was “worried, very worried” that the party was heading towards a repeat of the 2016 election in which Trump snatched an unexpected victory partly because of the softness of the African American vote.

About 4.4 million voters who voted for Barack Obama in 2012 stayed home in 2016. More than a third were black.

“There would be a President Hillary Clinton right now if the African American turnout had been close to what it had been in 2012,” Booker said. “That’s how real is the power and influence of the Democratic party’s most loyal voting base.”

We have to make sure there’s a candidate on that stage that black and brown people in this country can trust Cory Booker

He added: “That’s why we have to make sure there’s a candidate on that stage that black and brown people in this country can trust, in whom they see their lived experience.”

The issue of the fading diversity in the Democratic race has become a major talking point in the wake of the California senator Kamala Harris dropping out for lack of funds. With Harris out, the spotlight is increasingly falling to Booker. He has been quick to sound the alarm over the consequences of black voters feeling undervalued as election year approaches.

Asked what message an all-white stage would send African Americans, he told the Guardian: “The message is already being sent.

“I’ve talked to civil rights leaders, Congressional Black Caucus members, you hear this being talked about now in the black community. People are saying there, ‘This can’t be,’ especially when there is a candidate out there who is fully qualified under any objective criteria other than the arbitrary polling system.”

Booker has met the bar of 200,000 unique donors set by the Democratic National Committee (DNC) but is falling short of attaining 4% in four national or early state polls. Unless he can do that by Thursday he will not have a place at the debate.

Cory Booker speaks to a crowd about gun violence and white nationalism in Charleston, South Carolina. Photograph: Sean Rayford/Getty Images

To rub salt into the wound, Tom Steyer, a billionaire hedge fund manager, has secured a position. Booker said the presence of billionaire candidates in the Democratic race – Michael Bloomberg is the other – was an insult to “voters who wonder how you can have talented, qualified, experienced, proven diverse candidates that aren’t on the stage.

“We’ve seen how you gin your poll numbers up by running nonstop ads – that shouldn’t be the decider of who’s on stage at the debate, it sends a very bad signal.”

The Guardian asked Steyer what he thought of the argument that the race was being distorted with billionaires buying prominence while diverse candidates languished.

He said: “I’m concerned about the diversity in the debates, too, and I have asked the DNC to change the criteria of the debates to get more diversity.”

It’s important we have a diverse group of people competing … and I don’t think it’s fair, but I don’t run the process Tom Steyer

Steyer has been able to use his personal wealth – he is worth $1.6bn according to Forbes – to vastly outspend Booker so far, buying $55m of TV and online ads to Booker’s $3m. The disparity is paradoxical given that one of Steyer’s main political platforms is combatting growing inequality.

“A lot of people have complained to the DNC about how this is going,” Steyer said. “It’s important that we have a diverse group of people competing for the nomination of the Democratic party and I don’t think it’s fair, but I don’t run the process.”

The thorny question of billionaires using their financial muscle to wrestle themselves into the Democratic race has welled up again with the late entry of Bloomberg. The former New York mayor is outspending all the top-tier candidates combined, according to the Washington Post.

It did not soothe the increasingly fractious mood when Bloomberg commented that Booker was “well spoken”. He later apologised.

Booker carved out his political reputation as mayor of Newark, New Jersey. He has a distinguished resume that includes having been a Rhodes scholar at Oxford, though he has complained that the media rarely point that out, unlike his Democratic rival Pete Buttigieg, also a Rhodes scholar.

Joe Biden and Kamala Harris on the second night of the second Democratic debate in Detroit, in July. Photograph: Lucas Jackson/Reuters

Booker said he was still confident he would make the debate later this month, joining those who have already been guaranteed a place: Buttigieg, Steyer, Joe Biden, Elizabeth Warren, Bernie Sanders and Amy Klobuchar.

The decision of more than a million African American voters to stay home rather than vote in 2016 is widely considered an important factor behind Trump’s shock victory. Trump won the presidency comfortably in the electoral college yet in the key states of Michigan, Pennsylvania and Wisconsin he beat Clinton by only 77,000 votes.

In the Wisconsin city of Milwaukee alone, Clinton attracted 70,000 fewer black votes than Obama in 2012.

Booker said his anxieties about a potential repeat next November did not stop at the White House. He said low African American turnout could also have an effect on senatorial races in North and South Carolina, Georgia and Arizona that would prevent the Democrats taking back the Senate.

“I’m very worried about consequences for the US Senate – it’s not just Donald Trump,” Booker said. “We cannot win in these very diverse states without not just good turnouts of African Americans – we need Obama’s record turnouts again.”

Booker was speaking at a Democratic presidential forum in Cedar Rapids, Iowa organized by the Teamsters and focusing on workers’ rights. The Guardian and The Storm Lake Times were media partners of the event.