MILWAUKEE When Sen. Elizabeth Warren (D-Mass.) came here to campaign for Lt. Gov. Mandela Barnes on Saturday, she laid out three reasons she was backing him for Wisconsins Democratic nomination for Senate: his support for abortion rights, his vision on combating climate change and his background as the son of two union members.

But she saved the most important reason for last.

Mandela can win, Warren told a crowd of about 100 people gathered on the patio of a lakefront bar. Mandelas the one who can take it to Ron Johnson.

The outcome of the August 9 primary might depend on how many Wisconsin Democrats agree with Warrens assessment of Barnes, a 35-year-old former activist and progressive state legislator who is the first Black man ever elected to statewide office here.

Barnes is one of three Democrats in the running to battle Johnson, a businessman and Republican who has increasingly embraced conspiracy theories as he runs for a third term in the Senate. Barnes has led most public surveys of the race against Alex Lasry, a businessman whose father owns the Milwaukee Bucks, and state Treasurer Sarah Godlewski. A fourth candidate, Outagamie County Executive Tom Nelson, dropped out of the race on Monday and endorsed Barnes.

In-state Democrats view Barnes as the favorite in the primary, with Lasrys ability to fund his campaign hes already loaned his campaign at least $12.3 million, according to Federal Election commission records giving him the best shot at an upset.

Throughout the contest, however, some Wisconsin Democrats have quietly expressed doubt about Barnes ability to win in November, fearing he is too liberal on issues like criminal justice and immigration to triumph against Johnson. Barnes supporters have responded with skepticism of their own, arguing that Lasrys out-of-state roots, and his and Godlewskis wealth, make them unable to draw sharp contrasts with the incumbent.

People are tired of sending people to Washington who dont vote in their interest, who dont advocate on their behalf, Barnes told HuffPost. The Senate is broken. Its a club for millionaires.

The debate is reminiscent of the 2020 presidential primary, where college-educated white voters in particular fretted and speculated about various candidates ability to beat then-President Donald Trump arguing about how ideology and demographic background might influence swing voters or turn out reluctant ones. Those electability debates seemed to hurt female candidates like Warren and minority candidates like Sen. Cory Booker (N.J.), who has also endorsed Barnes.

Thats not necessarily the case this time around. At a Barnes event with labor supporters at a brewery the night before Warrens visit, multiple union leaders said Barnes own working-class roots his mother was a public school teacher, his dad a member of the United Auto Workers would appeal even to their unions conservative-leaning members as he battles ultra-wealthy candidates in both the primary and general elections.

Hes a working-class candidate, hes got the lived experience. Hes not looking down on us from above, said John Drew, the vice president of UAW Local 72. Theres a conservative attitude that you need a boring white guy, that people wont vote for a Black candidate, but I dont buy that.

The outcome of the race is crucial: Democrats control the 50-50 Senate only because of Vice President Kamala Harris ability to break ties, and Wisconsin represents the partys most realistic opportunity to oust an incumbent. A victory over Johnson, a Trump loyalist known for his brashness, would limit Republican chances to take control of the Senate, or at least shrink the size of a GOP majority.

The importance of the contest, along with the narrow, upset nature of Johnsons wins over former Sen. Russ Feingold (D) in 2010 and 2016, has Democrats in the state anxious about making the right choice. In interviews, none of the three leading candidates were willing to say their opponents would be able to beat Johnson in November.

I just know that my campaign is the strongest against Ron Johnson, Barnes said.

I think were the best one to be able to win, Lasry said.

I think thats something voters are going to have to look at when theyre making their decision, Godlewski said.

There have been just three public surveys of the race, all from Marquette University Law School. The most recent, conducted in June , showed Barnes with 25% of the vote, Lasry with 21% and Godlewski with 9% and more than a third of the electorate undecided. None of the candidates were well known, with more than half of registered voters in the state saying they didnt know enough about any of the three to form an opinion. And all three polled in tight, margin-of-error races against Johnson.

With the exception of a handful of attacks from Nelson directed at Lasrys wealth, the race has been more passive-aggressive than nasty, the contrasts between the candidates left implicit rather than spoken aloud.

Lasry, sitting at the citys fairgrounds during a food truck festival as his 11-month-old daughter ate cheese curds, never went after Barnes by name while talking to reporters. He did draw a not-so-subtle stylistic contrast, comparing himself to Barnes boss in Madison, the famously conventional Gov. Tony Evers (D).

Tony embodies the politics people are looking for, Lasry said, noting his own successes in building a new arena for the Bucks. Its not this abrasive, aggressive, polarizing politics. Theyre looking for people who are actually trying to solve things.