On the first day of early voting in Wisconsin, Tim Walz called Elon Musk a “dipshit” while Barack Obama said of Donald Trump: “You’d be worried if Grandpa was acting like this.”

Both were speaking at a rally in Madison, a growing Democratic party stronghold, to encourage early voting and warn of the perils of a second Trump presidency. Obama went on to campaign for Kamala Harris in Detroit on Tuesday evening, alongside rapper Eminem, in an effort to drum up support in Michigan where polls suggest Harris and Trump are in a virtual deadlock.

The Democratic vice-presidential candidate ripped into Trump ally and Silicon Valley billionaire Musk, warning that he could be charged with regulating his own businesses if Trump were elected. Musk has also promised the chance to win $1m to voters in swing states who sign a petition linked to efforts to return Trump to power.

Walz also slammed Trump, who this week served meals at a McDonald’s in Pennsylvania, accusing him of “cosplaying” as a working-class person and noting that the restaurant had closed to accommodate the presidential candidate. “It was a stunt,” said Walz. “Fake orders for fake customers.”

“He is not the 2016 Donald Trump,” said Walz, describing Trump’s promise to prosecute his political enemies. “He’s talking about sending the military against people who don’t support him. He’s naming names.”

Obama, who won in Wisconsin in 2008 and 2012, urged his Madison audience to get to the polls and spent much of his speech attacking Trump.

“I wouldn’t be offended if you just walk out right now and go vote,” he said.

“When he’s not complaining, he’s trying to sell you stuff,” he added, referring to Trump, who has raised funds by selling gold-colored sneakers, bibles and $100,000 watches. “Who does that? You’re running for president, and you’re hawking merchandise.”

He compared Trump’s meandering rhetorical style with that of Fidel Castro, the former Cuban head of state who was known to deliver hours-long speeches.

“He calls himself the father of IVF. I have no idea what that means – you don’t either,” said Obama, casting Trump’s rambling speeches and sometimes confounding remarks as a sign of mental deterioration.

“You’d be worried if Grandpa was acting like this,” said Obama. “But this is coming from someone who wants unchecked power.”

Obama also acknowledged that while his signature healthcare bill, the Affordable Care Act, did not fix American healthcare, its passage meant people with pre-existing conditions are more able to access health insurance.

He spoke about efforts by his administration to implement a pandemic-preparedness plan and accused Trump of abandoning the effort, resulting in more Covid-19 deaths.

“Most of you know somebody whose life was touched,” said Obama, urging voters who are fed up with politics to participate in the November election anyway.

Before Walz and Obama spoke, Madison mayor Satya Rhodes-Conway, Representative Mark Pocan, Governor Tony Evers and Senator Tammy Baldwin – herself up for re-election on 5 November – encouraged voters to return their absentee ballots or vote absentee in person.

“Don’t take the risk of forgetting to vote– vote early,” said Pocan. “With the Packers game on the Sunday afternoon before the election, you can have a two-day hangover and not worry about missing the vote.”

More than 18 million people in the US have voted early so far in the 2024 election, with a little more than 326,000 of those coming from Wisconsin as of 21 October, according to the University of Florida’s Election Lab. Those numbers will increase dramatically now that Wisconsin’s early voting period has begun.

Since the 2020 election, when Trump cast doubts on the integrity of absentee voting amid the Covid-19 pandemic, early voting has been a source of consternation in the Republican party. After Trump lost the 2020 election and Republicans failed to generate a red wave during the 2022 midterm elections, GOP leaders have sought to encourage their base to cast ballots before election day.

Trump, who discouraged absentee voting before the 2020 election, has struggled to stay on message about early voting, alternately urging supporters to vote early and casting aspersions on the voting method – sometimes during the same speech.

With polls showing Harris and Trump in a dead heat across the swing states, including Wisconsin, the last-minute push to turn out voters could determine the outcome of the election. In 2020, Joe Biden won in Wisconsin by about 20,000 votes; in 2016, Trump beat Hillary Clinton in Wisconsin with a similarly slim majority. With 10 votes in the electoral college, Wisconsin will play a critical role in determining the outcome of the 2024 presidential election.

Deb and Rod Merritt, a retired couple from Sauk county, Wisconsin, who attended the rally on Tuesday, said the pressure of Wisconsin’s close margins and the extra time afforded by retirement drove them to volunteer for the Harris campaign.

“I’m definitely nervous,” said Deb Merritt, who said knocking on doors in the bellwether county – Sauk county voters have aligned with the winner repeatedly in presidential elections – was gratifying.

“We saw a few [undecided voters], mostly leaning Democrat,” said Rod Merritt. “Some people would say: ‘I’m voting for Kamala and my husband was for Trump, but he’s not going to vote.”

In both 2016 and 2020, Trump performed better in Wisconsin than polling suggested.

“We don’t know if that’s going to happen again this time, or which direction it’ll be or how big the error will be, but we have to expect that we need to overshoot to be able to win by a hair,” Ben Wikler, the Wisconsin Democratic party chair, told the Guardian. “For anyone who’s knocking on doors, if you think for a second you’ve got it in the bag, then go and sign up for another volunteer shift to drive it even higher.”

In Detroit, an energetic Obama performed part of an Eminem rap when he took the stage and then praised Harris as “a leader who has spent her life fighting on behalf of people who need a voice, need a champion – somebody who was raised in the middle class”. Reviving earlier jabs against Trump, he noted Harris “did not pretend to work at McDonald’s when it was closed”, but actually held a fast-food job in college to help with her expenses.

For his Michigan audience, Obama recounted the chaos Trump helped cause in the state after the 2020 election: “Because Donald Trump was willing to spread lies about voter fraud in Michigan, protesters came down, banged on the windows, shouting, ‘Let us in. Stop the count.’ Poll workers inside being intimidated … all because Donald Trump couldn’t accept losing.”