The Wisconsin Democratic primary isn’t until Tuesday, but after three of his top rivals dropped out of the race last month, Lt. Gov. Mandela Barnes is already sharpening his attacks against his Republican opponent, Senator Ron Johnson.In ads and speeches, Mr. Barnes has started hitting Mr. Johnson on what he calls a pattern of hurting the state’s manufacturing industry and failing workers. As he aims to make the race a referendum on Mr. Johnson, Mr. Barnes has his own vulnerabilities, and Republicans are certain to try to portray him as too left wing for Wisconsin.But his strengths, and Mr. Johnson’s own polarizing qualities (he has pushed false theories about the coronavirus pandemic and doubts about the 2020 election), are setting up a race that could help decide control of the Senate.“What he pulled off is pretty impressive — to coalesce the entire field behind him in such a short time,” said Joe Zepecki, a Democratic strategist in Wisconsin. “He is getting a head start, and you have to sort of just tip your cap to him.”Mr. Barnes, 35, is entering the primary as the clear favorite after three of his main competitors dropped out of the race in the span of one week in late July: Alex Lasry, an executive with the Milwaukee Bucks; Tom Nelson, executive of Outagamie County; and Sarah Godlewski, the state’s treasurer. All three have endorsed him.To consolidate the support, Mr. Barnes, who is the state’s first Black lieutenant governor and would be its first Black senator if he were to win, ran a tight campaign squarely centered on jobs and rebuilding the middle class. In an interview, he said his campaign had benefited from the kind of coalition building he did as a community organizer. He also said it came down to a broader recognition among Democrats that the stakes are just too high for infighting.“This is about uniting the party, but it is also about uniting the state,” Mr. Barnes said.He leads in fund-raising and name recognition among his remaining rivals, including Kou Lee, a restaurant owner; Steven Olikara, a musician; Peter Peckarsky, an investigative reporter, lawyer and consultant; and Darrell Williams, a state emergency management administrator.He has racked up endorsements from both progressive Democrats like Senators Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts and Representative Alexandria Ocasio-Cortez of New York, as well as centrists including Representative James Clyburn of South Carolina.Wisconsin is one of the nation’s most fiercely contested political battlegrounds. Along with Michigan and Pennsylvania, it was one of the key states in the country’s industrial core that Mr. Trump won in 2016, shattering the Democratic “blue wall” and leading to his election.Democrats have since made somewhat of a comeback. In 2018, Tony Evers was elected governor and Senator Tammy Baldwin won re-election. In 2020, President Biden won the state by just over 20,000 votes.Understand the Aug. 2 Primary ElectionsWhile the Trump wing of the Republican Party flexed its muscle, voters in deep-red Kansas delivered a loud warning to the G.O.P. on abortion rights.Takeaways: Tuesday’s results suggest this year’s midterms are a trickier environment for uncompromising conservatives than Republicans once believed. Here’s what we learned.Kansas Abortion Vote: In the first election test since Roe v. Wade was overturned, Kansas voters resoundingly decided against removing the right to abortion from the State Constitution, a major victory for the abortion rights movement in a reliably conservative state.Trump’s Grip on G.O.P.: Primary victories in Arizona and Michigan for allies of former President Donald J. Trump reaffirmed his continued influence over the Republican Party.Winners and Losers: See a rundown of the most notable results.The Senate race is expected to be close. Mr. Johnson, 67, who is seeking his third term, is one of the most vulnerable Republicans this cycle. A recent Marquette University Law School poll showed he was viewed favorably by 37 percent of respondents and unfavorably by 46 percent. Additionally, Mr. Biden’s poll numbers are poor, and out-of-power parties typically perform well in the midterms during a president’s first term.Mr. Johnson has alienated many voters by suggesting that gargling with mouthwash could fend off Covid-19 and saying people who don’t like Wisconsin’s abortion laws can move. He has downplayed the U.S. Capitol attack, saying it didn’t “seem like an armed insurrection” and floating theories that Democrats edited videos to exaggerate the mob violence.And the House Jan. 6 committee this summer surfaced embarrassing evidence that Mr. Johnson wanted to hand-deliver fake elector votes from Michigan and Wisconsin to Vice President Mike Pence. Mr. Johnson acknowledged receiving the package but claimed he did not know where it came from or what it contained.But Mr. Barnes stuck to economic issues in his first attack television ad against Mr. Johnson, echoing his message throughout the Democratic primary. The ad criticizes Mr. Johnson for publicly praising outsourcing and defending a company that moved jobs to China from Wisconsin. On the trail, Mr. Barnes has been criticizing the senator over his comments suggesting Social Security and Medicare should be eliminated as federal entitlement programs and instead should be approved annually by Congress.Senator Ron Johnson has faced scrutiny for controversial remarks he made regarding the U.S. Capitol attack, Covid-19 and Wisconsin abortion laws. Sarahbeth Maney/The New York TimesMr. Johnson has pushed back against the criticism and has argued he stood up for small businesses when he pushed for a tax provision in the 2017 Republican tax law to level the playing field for them. “A manufacturer himself, Ron Johnson helped Wisconsin small businesses remain competitive with the big guys by making sure they got a tax cut that helped businesses all across the state survive the pandemic,” said Ben Voelkel, a spokesman for Mr. Johnson, pointing to the measure.The provision also benefited his family-run plastics company.Independent fact checkers have found that the claim from at least one Democratic group arguing Mr. Johnson’s vote for the law rewarded “companies that outsource to China” was false, and a 2021 study by university researchers found the law decreased incentives for U.S. firms to move operations out of the country.But fact checkers have also found Mr. Johnson’s tax provision overwhelmingly benefited ultrawealthy Americans over small businesses.Mr. Barnes has been the target of criticism as well. He has been cited for paying his property taxes late, and Republican activists and local leaders have sought to paint him as a far-left Democrat who supports stances like abolishing U.S. Immigration and Customs Enforcement.“Mandela Barnes will speak out of both sides of his mouth to convince voters that he is a moderate,” said Mark Jefferson, the executive director of the Republican Party of Wisconsin.At one point, Mr. Barnes was photographed holding an “abolish ICE” shirt, though he has said that is not his position. More recently, he opposed the Biden administration’s proposal to end Title 42, a Trump-era policy that was introduced during the pandemic and has been used to turn away most migrants at the U.S.-Mexico border. Mr. Barnes argued that Mr. Biden should first have a comprehensive plan to handle an increase in people crossing the border.Mr. Barnes, whose father worked third shift at a General Motors factory and whose mother was a longtime schoolteacher, is betting his record and biography will help him weather the attacks. He became a community organizer after watching former President Barack Obama speak at the 2004 Democratic National Convention.By the time he was 25 in 2012, Mr. Barnes had defeated an incumbent state lawmaker in Milwaukee, his hometown. He lost a Senate bid four years later, but he won his race to become lieutenant governor in 2018 with Mr. Evers at the top of the ticket and a strict focus on the economy.Katie Rosenberg, the mayor of Wausau, Wis., said Mr. Barnes was talking about the issues residents care about, including affordable child care and health care, the expansion of broadband and the need to stop the corporate takeover of family farms. Earlier this year, the two visited small businesses wracked by the pandemic. They even got tattoos in support of a local tattoo parlor. Mr. Barnes featured it on his TikTok.“I am an optimist,” Ms. Rosenberg said. “I think he can do this. He has a lot of momentum.” More