As former President Donald Trump returns to Wisconsin on Tuesday night for the first time in nearly two years, Democrats have a close eye on him – but not only him – as they build a general election campaign in this critical battleground state.

“Any appetite for Robert Kennedy Jr. this fall?” progressive host Mike Crute asked this week on “The Devil’s Advocate Radio Show,” where listeners call in and compare notes about liberal politics. “Would you consider voting for a third party?”

As Wisconsin voters cast ballots Tuesday in a largely symbolic primary seven months ahead of the November election, that question is on the minds of Democrats and Republicans as they brace for a rematch between President Joe Biden and Trump – with Kennedy’s independent candidacy increasingly becoming a worrisome wild card in the race.

“Third-party candidacies, to my opinion, are an unacceptable answer in the 2024 election,” Crute said, before delivering an even sharper message to his audience: “Don’t throw away your vote.”

Look no further than Wisconsin to see how serious the threat of third-party challengers could be to the Biden-Trump rematch. In 2020, Biden won the state by fewer than 21,000 votes, with no Green Party candidates on the ballot. Four years earlier, Trump carried Wisconsin by nearly 23,000 votes, with the Green Party’s Jill Stein earning more than 30,000 voters. Stein, along with Kennedy and left-wing scholar Cornel West, are offering themselves up this year as alternatives to voters unimpressed – or even angry – over the prospect of choosing again between two of the least popular candidates in modern times.

“I would say Jill Stein, the Green Party, worries me as much as Kennedy does,” said Crute, who broadcasts his progressive show across the state. “But I’m concerned. Anything that changes the head-to-head electoral math between Trump and Biden and I don’t like that.”

In an interview on CNN’s “Erin Burnett OutFront” on Monday, Kennedy downplayed concerns about potentially spoiling the election for Trump. He argued that neither Biden nor Trump will advance policy goals he wants prioritized, such as reducing the national debt and the defense budget.

“I don’t think either President Trump or President Biden are going to solve the debt crisis in this country, which is existential,” Kennedy said. “I don’t think either of them are going to get us out of foreign wars, this addiction that we have to forever wars.”

Kennedy’s peace message appears, for now, to be more of a concern for Democrats, who are increasingly divided over Biden’s support for Israel’s unrelenting war in Gaza. Though Biden and Trump have both clinched their respective party’s nominations, progressives are again urging Democrats to lodge protest votes – this time for “uninstructed” – as a warning to Biden.

A national poll from Marquette University Law School, conducted in February, highlighted the volatility of the coming general election. The survey found Trump with a lead of 51% to 49% over Biden among registered voters, well within the poll’s margin of error. When third-party candidates were added to the mix, both Biden and Trump saw their numbers fall by about 10 percentage points. Trump received 42% to Biden’s 39%, while Kennedy received 15%, with West and Stein taking 5% combined – meaning as many as 1 in 5 voters seem open to casting a third-party ballot.

As Kennedy seeks to qualify for the presidential ballot in states across the country, his Wisconsin supporters are taking steps to organize a summertime push to gather the 2,000 signatures needed here – a far lower bar than many other states. State law requires signatures to be gathered between July 1 and August 6.

Phil Anderson, a longtime Wisconsin Libertarian who is supporting Kennedy’s candidacy, said he believes widespread discontent in the electorate over the notion of a Biden-Trump rematch creates a greater opening for a third-party candidacy.

“It really brings forward the idea that the two big parties just hand us candidates to choose from,” Anderson said. “Even if they’re somewhat sympathetic to Trump or Biden, they realize that the game is rigged and their voice isn’t being heard.”

If Kennedy is on the November ballot, Anderson said, he has little doubt that he siphons support from both Trump and Biden.

“The fact that he’s a protest candidate means something,” Anderson said. “There will be people that go in to vote and they see Trump and Biden and their stomach turns a little bit and they’re like, ‘Kennedy has to be better.’ Even if they don’t know that much about him, they’ll feel like he has to be better.”

Brian Schimming, chairman of the Wisconsin Republican Party, said he believed the prospect of Kennedy and other third-party candidates making the November ballot posed a bigger challenge to Biden than to Trump.

“There are some issues where he has some appeal, probably, to our base,” Schimming said. “But when it gets down to the 10 days before the election, is he more likely to hurt Democrats than us? I think no question.”

For now at least, most surveys suggest the Republican base is more energized than the Democratic base this year. Schimming said that energy converts into more loyalty for Trump than Biden.

“Our base is more likely to stick,” he said. “With the president’s weakened base, having an option like Robert F. Kennedy (Jr.) is more trouble for them on that level than it is for us.”

As the Democratic National Committee mounts a robust challenge to Kennedy’s candidacy, putting teams of lawyers in place to scrutinize ballot applications across the country, Democratic leaders in Wisconsin acknowledge the uncertainty third-party candidates can bring.

“I’m hopeful that we have all learned our lesson and we really can’t take the risk of a second Trump administration,” said Kelda Roys, a Democratic state senator from Madison, who added that Hillary Clinton’s loss in 2016 remains seared into the minds of most Democrats here.

Stein has often argued that voters who turned out for her in 2016 would have stayed home, rather than voted for Clinton, if she did not run. But Democrats have long viewed Stein’s campaign as one of the decisive factors in delivering Trump the state of Wisconsin and the presidency.

“It’s going to be about whether or not we can make sure that people know about President Biden’s accomplishments,” Roys said. “But we cannot afford to take the risk. Everybody in Wisconsin is very acutely aware of how close our elections are, and that’s why we’re not taking anything for granted.”

As Democrats gathered Monday night for a neighborhood canvass on the southwest side of Madison, Anthony Gray, a member of the Dane County Board of Supervisors, said it was difficult to get a sense of the true impact of a third-party candidate.

“I’m not panicked yet. Let’s see how the third parties develop,” Gray said. “But Bobby Jr. is the one I’d be most concerned with.”