Kamala Harris and Donald Trump have been ramping up personal attacks on each other as the White House rivals blitzed battleground states 16 days before the election.

In Pennsylvania, Trump served fries at a McDonald’s as he sought to cast doubt on Harris’s biographical detail about having worked decades ago at the fast-food chain.

The US vice-president was in Georgia, where she tweeted that Trump was “exhausted, unstable, and unfit to be President of the United States”.

Polls show the two locked in a razor-tight race across the country, including in the seven battleground states that could swing the election.

Speaking in traditionally Republican-leaning Lancaster County on Sunday afternoon, Trump focused heavily on the economy and immigration – issues his campaign believes give him an edge with undecided voters.

“If you look at the polls, the biggest thing is the economy,” he said. “But I think this [the border] is bigger than the economy… that’s the number one thing people want to talk about.”

Earlier in the day, Trump visited a McDonald’s in Feasterville-Trevose, a suburb of Philadelphia, where he learned to make fries, dunking the wire basket in sizzling oil, and serving meals at a drive-through window.

The restaurant itself was closed to sit-in diners during Trump’s visit.

“I like this job,” said the Republican, who is himself fond of Big Macs and Filet-o-Fish sandwiches.

He again accused Harris of “lying” about having once worked at the fast-food chain.

“I’ve now worked [at McDonald’s] for 15 minutes more than Kamala,” said Trump.

Harris spokesman Ian Sams told the BBC that Trump’s stunt was a sign of “desperation”.

“All he knows how to do is lie,” he told the BBC. “He can’t understand what it’s like to have a summer job because he was handed millions on a silver platter, only to blow it.”

The campaign added that the vice-president had worked on the cash register, ice cream machine and fry machine at a McDonald’s on Central Avenue in Alameda, California, in the summer of 1983.

McDonald’s placed an ad in the 1983 edition of the yearbook at a local high school, with pictures featuring a couple of students who worked there at the time.

The BBC has spoken to one of the students in the photos, who said he remembers a lot of people who worked alongside him at the restaurant more than 40 years ago, though he did not recall Kamala Harris being one of them.

The New York Times, however, spoke to a high school friend of Harris, Wanda Kagan, who said she remembered the now-vice-president working at McDonald’s around that time.

On Sunday, Harris celebrated her 60th birthday and gave an interview to MSNBC, whose host asked her about Trump’s remark at a rally in Latrobe, Pennsylvania, on Saturday that she was a “[expletive] vice-president”.

Harris said Trump’s language “demeans the office” of the presidency. “He has not earned the right” to be president again, she added.

Earlier in the day, she told a congregation of black church-goers near Atlanta, Georgia, to vote against “chaos, fear and hate” – personified, she suggested, by her Republican rival. On Saturday she told a rally in the city that Trump was “cruel”.

Meanwhile, Elon Musk, the billionaire founder of Tesla and SpaceX who has been campaigning for Trump, pledged to give away $1m a day to voters who sign his petition backing the US Constitution.

Pennsylvania Governor Josh Shapiro, a Democrat, said the plan was “deeply concerning” and that law enforcement should potentially look into the matter.

The crowd at Trump’s town hall-style event in Lancaster on Sunday was largely receptive to his remarks.

But one attendee, Jordan Ashby, told the BBC he was undecided and waiting to hear closing arguments from both candidates.

“I truly don’t know yet,” said the Lancaster resident when asked how he would vote. “I have family on both sides of the fence. It’s a difficult time right now.”

Other attendees told the BBC they were attracted by Trump’s vows to secure the US-Mexico border and reduce inflation.

“[Inflation] is what I see a lot of in my daily life, especially with gas prices and affording groceries,” said Brennan Zeyak, an 18-year-old college student voting for the first time this year.

Both candidates will continue making their final pitches on Monday.

Harris will on Monday embark on a whirlwind tour of Pennsylvania, Michigan and Wisconsin.

Trump will head to the hurricane-stricken town of Asheville, North Carolina, before holding a rally in the town of Greenville.