After months of flinging mud, Senator Ron Johnson was finally obliged to admit that his Democratic opponent in the upper midwestern state of Wisconsin had never actually made a call to “defund the police”.

But that did not stop the Trumpist senator’s re-election drive from continuing to broadcast racially charged advertisements falsely claiming that Mandela Barnes, the lieutenant governor of Wisconsin, “rationalized violence” against the police and tying him to the most controversial positions of Black Lives Matter.

Barnes and his supporters dismiss the ads as evidence of Johnson’s desperation. But the campaign of “race and fear” has had an impact as an election that Barnes once looked to have in the bag is now too close to call.

Across the country, Republican strategists have ratcheted up attacks on Democrats over fears of crime as the midterm elections approach with predictable results in many races. But Barnes, who is running to become Wisconsin’s first Black senator and is named after South Africa’s iconic former president, is on the end of a particularly pointed campaign that has eaten into a once substantial lead in the final weeks of a race that could decide control of the US Senate.

“There’s definitely a racial overtone,” said Charles Franklin, director of the respected Marquette law school polling of Wisconsin voters.

“The massive amount of negative advertising attacking Barnes on crime more than anything else is surely the explanation for why he has seen the gap close since August, or a big part of it.”

Johnson won the seat, once held by the notorious communist baiter Jospeh McCarthy, in the 2010 backlash against Barack Obama’s presidency, unseating a three-term Democrat. He was re-elected in 2016 by a margin of just 3.4 points.

Earlier this year, the Cook Political Report rated Johnson one of the most vulnerable incumbent senators in part because of association with attempts to submit a slate of fake electors to overturn Biden’s election victory, his promotion of conspiracy theories around the January 6 insurrection at the Capitol, and for his support of a total ban on abortions. It now says the seat is a toss-up.

Barnes has seen a seven-point lead evaporate in recent weeks amid a barrage of negative advertising largely funded by two billionaires who the Democrat’s campaign say are rewarding Johnson for his support of tax cuts that benefited them by hundreds of millions of dollars.

Diane Hendricks, a rightwing billionaire businesswoman and Wisconsin native closely tied to Donald Trump, and Richard and Elizabeth Uihlein, founders of the Wisconsin-based Uline packaging company who have a long history of funding far-right candidates, are the main donors to a political action committee, Wisconsin Truth, which until recently was heavily outspending the Barnes campaign.

Franklin said his most recent poll showed the two candidates both at 47% support among registered voters because significant numbers of independents, who were neutral on Barnes in August, turned against him amid the barrage of attacks ads over crime.

“When we look across our surveys, it looks like that wave of advertising from August to September raised the salience of the issue, especially with independents,” he said.

Wisconsin Truth has portrayed Barnes as supporting radical reform of the police and the scrapping of the US immigration agency because he is supported by groups that back those positions even though he has repeatedly said he does not. But key to the ads are their racial overtones.

One of them shows a picture of Barnes with three members of “the Squad” of congressional progressives, all women of color – Alexandria Ocasio-Cortez, Ilhan Omar and Rashida Tlaib. The text says: “Mandela Barnes, different”. The word different then changes to “dangerous”.

Criticism has also been made of another ad, paid for by Wisconsin Truth, that includes footage of “actual crime scenes”. In one scene, a person apparently committing a crime is circled in red at the same time as Barnes’s name appears on the screen, seeming to link the two.

Some of the advertising has darkened Barnes’s skin in what would appear to be an attempt to make him appear menacing to some white voters.

A Wisconsin state representative, Evan Goyke, told the Milwaukee Journal Sentinel that the ads were “despicable” and accused Johnson and his allies of using “race and fear as their main election tactics”.

Johnson has pressed similar claims in televised debates including asserting that Barnes “has a record of wanting to defund the police” because he proposed spending some of the police budget on social workers to assist frontline officers in dealing with some crisis situations, such as those involving the mentally ill and homeless. The Republican senator was later forced to acknowledged that Barnes hadn’t said he wanted to “defund” the police but still accused him of using “code words”.

Barnes said it was a bit rich for Johnson to claim to defend the police when he backed Trump supporters who stormed the Capitol last year.

“I won’t be lectured about crime from somebody who supported a violent insurrection that left 140 officers injured,” he said.

On the campaign trail, Barnes told the Guardian he was not surprised by Johnson’s claims.

“We knew they will run bad-faith attacks because that’s all they have. Senator Johnson doesn’t have a record to defend so all he can do is just try to lie and distract and make up things about me. And that’s the worst part about this,” he said.

Barnes’s campaign included a stop at the King Solomon Missionary Baptist church in Milwaukee where the pastor, the Rev Charles Watkins, gave a pointed sermon about how voters’ perceptions of his city’s Black neighborhoods were shaped by intense coverage of crime while ignoring more positive aspects of the community – a balance he said was not replicated in coverage of white neighborhoods.

Asked by the Guardian if Johnson’s supporters were running a racist campaign, Watkins paused.

“I don’t want to say that. I don’t want to, but some of the things that the other candidate has said have been racist. Just like when he said during the insurrection on January 6 that he would have been more fearful if it had been Black Lives Matter. For him to say that, yes, that’s racist,” he said.

Watkins said he thought the negative campaign was having an impact.

“If you say it long enough and loud enough, people will believe it,” he said. “It’s fear tactics. Not one time did Mandela Barnes ever come out and say ‘defund the police’. Not one time. That’s one of the reasons why he’s behind in the polls. People look at that because they’re not really looking at the person.”

Matt Mareno, the chair of Waukesha county Democrats just west of Milwaukee, said he sees that on the doorstep. He said that campaigners for Barnes were forced to spend time explaining that the claims made about him are false, making it harder to promote his policies to protect social programs and union rights, revive manufacturing in the state, and help family farms.

“We found when we’re talking to voters on the doorstep the only things they know about Mandela are the things they hear on TV. So they assume he is pro-crime, whatever that means, and that he wants to let all criminals out and cause mayhem in the streets. So for us it’s been a lot of having to introduce Mandela Barnes to a lot of these people, and explain complex policies when a lot of the world operates on bumper stickers,” said Mareno.

“They are blowing all the dog whistles they can because in a state like Wisconsin if you blow those dog whistles and get maybe the half percent of people who will be motivated to vote by race, that could be the difference between winning and losing. Half of our statewide elections are decided by less than one per cent.”

Johnson has responded to the charges of a bigoted campaign by accusing Democrats of “playing the race card”.

“That’s what leftists do,” he told Milwaukee talk radio.

Still, Franklin said that while Barnes had lost ground, and Johnson was slightly ahead among likely voters, the Democrat remains competitive in a state where elections frequently come down to the wire. Biden took Wisconsin from Trump in 2020 by just 20,000 votes – less than 1% of the ballot.

The Barnes campaign says that its funding has recently overtaken his rival and increased spending on campaign ads attacking Johnson for opposing abortion rights, including support for a federal ban that makes no exceptions for rape, incest or the life of the mother. Johnson told women that if they do not like Wisconsin’s restrictions on abortion then they should move to another state.

Franklin said polling shows that 81% of Democrats placed abortion rights among their most important voting issues after the supreme court struck down Roe v Wade, and that it has “clearly driven up enthusiasm for voting among Democrats” which will help Barnes.

Among them is Cara Adams, a store owner in Stevens Point in central Wisconsin. After Barnes stopped by her shop of locally made goods during a campaign tour, she said she wasn’t impressed by politicians but she was inclined to vote for him in large part because of abortion rights.

“I don’t affiliate with a specific party because politics is just gross. There’s just horrible political campaigns on both sides. Horrible, nasty things on TV,” she said.

“But Barnes is a lot more progressive in his thinking. I would be very uncomfortable if Ron Johnson came into my store today. His views upon women make me extremely uncomfortable, just knowing that any person feels like a woman has any rights less than a man is ridiculous.”

Adams said that she sees anecdotal evidence in her area that the supreme court ruling on abortion is stirring many women to vote who might not have taken an interest in the midterms.

That turnout is likely to prove key to Barnes if he is to overcome the barrage of negative advertising.

But Watkins, who works with Souls to the Polls to mobilise Black voters, said that even within Milwaukee’s African American community, in the face of a racist election campaign and amid fears that voting rights are likely to be further eroded in one of the most rigged electoral systems in the country, it can be a struggle to persuade people to vote.

“’We’re trying to get our community to understand that if your vote didn’t count, why are they trying so hard to take away your vote? They’re making it harder for absentee votes. They’re making it hard to register to vote. So something’s going on. What we are trying to do is wake up the community, to wake up the city. Let them know, hey, it do matter. Your vote do matter,” he said.