Barack Obama, who twice won Wisconsin by large margins, will travel to the battleground state in the final weeks of the current midterm elections, seeking to boost Mandela Barnes, the young lieutenant governor looking to unseat the Republican Ron Johnson in a key US Senate contest.

Barnes would be the first Black senator from Wisconsin. He held early leads over Johnson but the Republican, a prominent figure on the GOP hard right, has surged back. This week, a Marquette University Law School poll showed Johnson in the lead.

Barnes, who is from Milwaukee, has been trying to energize Black voters in a contest that could decide control of the Senate, which is currently split 50-50 and controlled by Democrats through the vote of the vice-president, Kamala Harris.

Obama, the first Black US president, is set to hold an early voting event on 29 October, less than two weeks before election day, in Milwaukee, the largest city in Wisconsin and home to its largest group of African American voters.

Politico reported this week that the Barnes campaign was reaching out to high-profile Democrats, seeking support as he slips in the polls. Joe Biden, Harris and Bernie Sanders were also named as potential guests.

Since his first run for the Senate in 2010, Johnson has marketed himself as a successful businessman upholding conservative values.

But he has leaned heavily into rightwing conspiracy theories around the 2020 presidential election and Covid-19 vaccines and remedies. In turn, his approval rating has dropped to 45%, the second-lowest for a Republican senator.

In a recent editorial, the Milwaukee Journal Sentinel, the largest newspaper in the state, called Johnson “the worst Wisconsin representative since the infamous Joseph McCarthy” and reminded readers he promised to serve no more than two terms, which he has now completed.

“Voters should hold him to that pledge in November,” the newspaper said.

But Johnson’s campaign has been gaining steam. Early polls showed Barnes with a slight edge but Johnson has launched an intense negative ad campaign, attempting to portray Barnes as bad for the economy in a time of high inflation and as a supporter for activists who want to defund police departments.

The lieutenant governor has not backed such campaigns.

At a heated debate last Thursday, Johnson, when asked to say something nice about his opponent, said that Barnes had loving parents and added: “What puzzles me about that is with that upbringing, why has he turned against America?”

Wisconsin has long been a swing state. After voting for Obama twice, Donald Trump beat Hilary Clinton in 2016 by less than one point. Biden won with a similarly small margin four years later.

The state is home to more tight races this midterm season, including the Democratic governor, Tony Evers, being challenged by Tim Michels, a construction company co-owner endorsed by Trump. Marquette polls have for months shown that race to be about even.

Tammy Baldwin, the state’s other US senator, and Gwen Moore, a congresswoman who represents Milwaukee, are also slated to appear with Democratic candidates for office, including the serving attorney general, Josh Kaul.