Donald Trump, the front-runner in the GOP presidential primary race, may not have gotten a welcome from union leaders as warm as President Joe Biden did in Michigan when he skipped the second GOP debate and instead addressed current and former union members outside Detroit.

But his decision is laying the groundwork for a 2024 general election battle over the working-class voters who helped propel him to the White House in 2016 but favored Biden in 2020. It's the clearest signal yet of the campaign's shifting focus to the general election and specifically a potential Trump vs. Biden rematch.

Ahead of Trump's visit -- which came just one day after Biden was greeted by UAW President Shawn Fain and made the unprecedented move of joining striking autoworkers on the picket line -- the president's campaign rolled out a new ad criticizing Trump's treatment of autoworkers. Titled "Delivers," the 30-second contrast spot is the Biden campaign's first to directly attack the former president.

The United Auto Workers backed Biden in 2020, but it hasn't made an endorsement yet for 2024. And despite Fain's criticism of Trump's planned visit -- which was announced before Biden's -- the former president's team believes he can drive a wedge between union leadership and the rank-and-file workers, many of whom supported him in 2016.

"The reality is that there's a disconnect between the political leadership of some of the labor unions and the working middle class employees that they purport to represent," Trump senior campaign adviser Jason Miller told CNN.

Trump allies began floating the idea of a visit shortly after the strike began, while his team was reaching out to Michigan Republicans to gauge interest.

Trump was expected to be well-received during his visit to Macomb County. The suburban county is historically a blue-collar stronghold where the "Reagan Democrat" voter emerged. Trump won it by about 11 percentage points in 2016 and 8 points in 2020. More recently, the county has been something of a battleground. In 2022, Democratic Gov. Gretchen Whitmer won the county by 5 points.

"Macomb County has a big batch of union members and they're a pro-Trump county, but not by much," said Barry Goodman, a former Michigan Democratic National Committeeman.

Biden won Michigan union households by 25 points in 2020, according to CNN exit poll data -- up from Hillary Clinton's 13-point advantage among them four years earlier.

But Trump's visit to Macomb, in particular, suggests the former president and his team see some of those voters as up for grabs in 2024.

"The people working on the floor -- blue-collar, average guys working hourly for the Big Three -- supported Donald Trump because they have traditional values, they own guns, they don't want their gun rights taken away or restricted. They're predominantly anti-abortion," said Brian Pannebecker, a staunch Trump supporter and president of Auto Workers for Trump, who rallied supporters -- both union and nonunion workers, including some UAW strikers.

Trump addressed some of those supporters Wednesday, when he spoke to autoworkers, plumbers, electricians, and current and former union members, including some UAW members and their families. Trump was expected to tout his administration's policies, including talking about bringing manufacturing jobs back to the United States in an economy-focused speech, sources familiar with the planning told CNN.

Trump allies were also encouraged by the timing of the speech, which they saw as more effective counterprogramming to the debate than his sit-down interview with Tucker Carlson that aired during the first Republican primary debate in August, two GOP strategists told CNN. "In Detroit, he'll actually be speaking to voters that he needs to win over," one of the strategists said.

Trump advisers told CNN they saw an opening with autoworker voters, in particular, because of Biden's push for electric vehicle production, which Trump has recently begun referring to as an all "electric car hoax," while also claiming it will move autoworker jobs overseas.

Early in his presidency, Biden announced a target that, by 2030, half of the vehicles sold in the United States would be battery electric, fuel-cell electric or plug-in hybrid, which would be a seismic shift for an auto industry dominated by gas-powered vehicles. Earlier this year, the Environmental Protection Agency released new proposed rules to speed up the process and ensure two-thirds of new cars sold in the US are electric by 2032.

Miller called the movement toward electric vehicles "a direct threat to every UAW worker in Michigan working on gas-powered vehicles."

The president of Drake Enterprises, the non-union auto parts manufacturer where Trump delivered his speech, echoed this sentiment. "If electric vehicles took over today ... we'd pretty much be out of business," Drake's Nathan Stemple told Fox News. "If all the trucks and vehicles went electric, we would be scratching for something to do."

Some members of the UAW have feared Biden's push for more electric vehicles could threaten its members' jobs since EVs require fewer people to assemble. Earlier this year, Fain publicly criticized Biden over his administration's financial support for such a transition. However, on Tuesday Fain told reporters that he believes a move toward electric vehicles does not hurt his union if "companies do the right thing."

Trump's rhetoric has little to do with the cause of the ongoing strike, which includes demands for wage hikes and a roll back of previous concessions.

Despite Trump's history of clashes with unions and his administration's policies that union leaders have called "pro-business," the former president has recently tried to cast himself on the side of autoworkers, while not weighing in directly on the strikers' specific wage concerns.

That hasn't sat well with union leaders. "Every fiber of our union is being poured into fighting the billionaire class and an economy that enriches people like Donald Trump at the expense of workers," Fain recently told CNN, responding to Trump's planned visit.

"We can't keep electing billionaires and millionaires that don't have any understanding what it is like to live paycheck to paycheck and struggle to get by and expecting them to solve the problems of the working class," he said in an emailed statement.

Biden does have the backing of the United Association of Union Plumbers and Pipefitters, whose president pegged Trump as out of touch on the issues important to union workers.

"After four years, one thing was clear: when it comes to the bread and butter issues our members care about -- fair wages, safe job sites, and the ability to retire with the dignity we earned -- Donald Trump is just another fraud," UA General President Mark McManus said in a recent statement.

That's the message Biden's campaign is leaning into in their new ad, which questions Trump's support for autoworkers and features footage of him golfing. "Manufacturing is coming back to Michigan because Joe Biden doesn't just talk, he delivers," the narrator continues, underscoring the importance of the pivotal battleground for the 2024 race.

This story and headline have been updated with additional details.