Alex Lasry, a son of the Milwaukee Bucks’ co-owner, ends his Senate bid in Wisconsin.

Alex Lasry, a Milwaukee Bucks executive who largely self-funded a Senate campaign in Wisconsin, plans to drop out of the Democratic primary on Wednesday, according to two people familiar with the decision, leaving Lt. Gov. Mandela Barnes as the favorite for the nomination to face Senator Ron Johnson.

Mr. Lasry, 35, whose billionaire father is a co-owner of the Milwaukee N.B.A. franchise, spent more than $12 million on his primary campaign but never eclipsed Mr. Barnes in polling. With less than two weeks to go before the state’s Aug. 9 primary, Mr. Lasry concluded he could not win the race, the person familiar with his decision said.

A formal announcement is planned for Wednesday afternoon outside the Bucks arena, where Mr. Lasry is expected to formally endorse Mr. Barnes, a campaign aide said. Mr. Lasry’s decision was first reported by The Milwaukee Journal Sentinel.

Another candidate, Tom Nelson, the Outagamie County executive, who ran a spirited but underfunded campaign, dropped out on Monday and endorsed Mr. Barnes. Mr. Lasry was Mr. Barnes’s chief rival for the nomination, though Sarah Godlewski, the state treasurer, and several other candidates remain in the race.

The primary was a relatively tame affair, with few negative attacks and little animosity between the candidates as they vied to face Mr. Johnson, a Republican loathed by the Democratic base for his amplification of false theories about the coronavirus pandemic and his efforts to overturn the 2020 election.

But Mr. Barnes, 35, has ample political vulnerabilities of his own. He has been cited for paying his property taxes late and has taken a variety of positions on immigration, at one point wearing an “abolish ICE” shirt and more recently opposing the Biden administration’s proposal to end Title 42, a Trump-era policy that was introduced during the pandemic and was used to turn away migrants at the Mexican border.