• President Joe Biden said Tesla and SpaceX CEO Elon Musk, now a Republican megadonor, launched his long career in the U.S. as an "illegal worker."
• Biden's comments about Musk followed a Washington Post report that cites correspondence, legal records and multiple people who helped Musk attain a work visa in 1996 after he had already been working here without one.
• Musk has become a regular campaign surrogate for Donald Trump in the presidential race against Vice President Kamala Harris.

President Joe Biden called out Tesla and SpaceX CEO Elon Musk, now a Republican megadonor and Trump campaign surrogate, for hypocrisy on immigration on Saturday, saying Musk launched his long career in the U.S. as an "illegal worker" before becoming the world's wealthiest man.

The president made these remarks at a campaign event to support Democrats that took place in Pittsburgh, Pennsylvania, on Saturday.

Labeling Musk as former President Donald Trump's wealthy new "ally," Biden said, "That wealthiest man in the world turned out to be an illegal worker here when he was here," referring to Musk.

"He was supposed to be in school when he came on a student visa. He wasn't in school. He was violating the law. He's talking about all these 'illegals' coming our way," Biden added.

He then criticized Trump and Republicans for failing to sign legislation that would fix "the problem with the border." He added, "We have fewer people crossing the border illegally now — or crossing the border period — than at any time since his third year as President of the United States."

The Trump campaign did not immediately respond to CNBC's request for comment on Biden's remarks.

Musk recently completed a series of "town hall" events in the same swing state of Pennsylvania, where he sought to convince voters to back Trump and Trump's policies. Musk also stirred up his fan base there by doling out $1 million lottery-style prizes to registered voters in swing states who signed a petition distributed by his pro-Trump group, America PAC.

According to an analysis by the Peterson Institute for International Economics, Trump's new immigration policy proposals include plans for the largest deportation operation in U.S. history, ending birthright citizenship, and revoking the visas of and deporting foreign students who are pro-Palestinian protestors, among others.

Biden's comments about Musk, his Trump alliance and hypocrisy on immigration followed a Washington Post report that cites correspondence, legal records and multiple people who helped Musk attain a work visa in 1996 after he had already been working here without one.

Musk had arrived in the U.S. with the stated intention of attending grad school at Stanford in the mid-90s. He did not enroll in the program to which he said he was accepted and instead began to build a venture-backed startup called Zip2 with his brother.

The Washington Post wrote that investors in Musk's first company worried about their "'founder being deported' and gave him a deadline for obtaining a work visa."

Zip2 sold for about $300 million in 1999, a windfall that enabled Elon Musk to later become an early investor in and chairman of Tesla, and to start his capital-intensive aerospace venture SpaceX, which is now a major U.S. defense contractor.

Those businesses have propelled Musk to become the world's wealthiest person on paper. According to Forbes, the Tesla CEO's net worth stands at around $274 billion today.

In late 2022, Musk used that considerable wealth to acquire the social network Twitter in a $44 billion buyout.

On the platform, since rebranded X, Musk has repeatedly claimed in posts seen by his massive online fan base that "open borders" and undocumented immigrants are somehow harming the United States.

He also has shared the false claim that noncitizens are systematically voting in U.S. elections, a conspiracy theory floated by conservative groups to lay the legal groundwork to contest the election results if the Democratic nominee, Vice President Kamala Harris, wins the presidency.

In the U.S., it's already a federal crime and a crime under every state's laws for noncitizens to register or vote in federal elections.

According to studies compiled by the Brennan Center for Justice, "extensive research reveals that fraud is very rare, voter impersonation is virtually nonexistent, and many instances of alleged fraud are, in fact, mistakes by voters or administrators. The same is true for mail ballots, which are secure and essential to holding a safe election amid the coronavirus pandemic."

— CNBC's Rebecca Picciotto contributed to this report.