Republican presidential nominee Donald Trump has shaken fries in the US election campaign as he stepped up his attack, accusing Vice President Kamala Harris of “lying” about previously working at McDonald’s.

Trump visited a McDonald’s in Pennsylvania on Sunday (US time), posing to work the fryer in the kitchen and hand orders to surprised drive-through customers.

Trump has been fixated in recent weeks on Harris’s summer job at the American franchise during her college years. Her campaign has highlighted it as part of her middle-class origins, in a pitch to those voters.

But Trump says he doesn’t believe Harris, and as recently as Friday told supporters at a rally in Detroit that she had “lied about working at McDonald’s”.

After escalating personal attacks on Harris on Saturday, Trump told Breitbart News he would work at a McDonald’s “just for fun” to mock Harris.

Stepping off his plane, Trump said: “I’m going for a job right now at McDonald’s.

“I really wanted to do this all my life.”

The Harris campaign responded on Sunday by counter-accusing Trump of lying.

“When Trump feels desperate, all he knows how to do is lie,” Harris campaign spokesman Ian Sams said.

“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.”

Trump was known to order McDonald’s to the White House when he was US president.

Footage of Sunday’s visit shows Trump standing at the drive-through window saying he has worked at the franchise “15 minutes more than Kamala”.

“She never worked here,” he says.

Asked why he thought his Democratic opponent was lying about working at the franchise in her youth, Trump says: “Because she’s ‘lying Kamala’ that’s why.”

Harris and Trump, who are essentially tied in the race for president in the most competitive states, have sharpened their attacks in recent days while drumming up support for early votes by mail or in person ahead of the November 5 election.

After being joined on Saturday in Detroit by hometown rapper Lizzo and in Atlanta by pop icon Usher, Harris got a birthday boost from another music legend, Stevie Wonder, on Sunday when he dropped into a church she was visiting.

Harris, who turned 60 on Sunday, and Trump, 78, are focusing on Pennsylvania, Michigan and a handful of other states likely to decide the election. Both parties have strong support in all the states and electoral results have been close in past cycles.

Harris and her running mate, Minnesota Governor Tim Walz, doubled down on Sunday on a “Souls to the Polls” initiative aimed at using gospel performances, national and state faith leaders, elected officials and others to mobilise voters.

Donald Trump surprises drive-through customers. Photo: Getty

Harris, who was raised in the teachings of the black church and sang in a church choir, attended a service and spoke at New Birth Missionary Baptist Church in Stonecrest, Georgia, about 50 kilometres east of Atlanta.

She also spoke at Divine Faith Ministries International in Jonesboro, just south of Atlanta, where she encouraged worshippers to exercise their voting rights. Wonder was there, and sang his version of Happy Birthday for Harris.

After her events, Harris was to record an interview with civil rights leader Al Sharpton to air on MSNBC later on Sunday.

Walz, meanwhile, was at a service in Saginaw, Michigan, and his wife, Gwen Walz, was visiting a church in Las Vegas.

Harris will need strong results in the majority non-white cities of Detroit and Atlanta and their surrounding suburbs to repeat President Joe Biden’s 2020 wins in Michigan and Georgia.

Trump also plans a rally in Lancaster, Pennsylvania.

He is seeking to take advantage of what he felt is an improved position for him in opinion polls that show a deadlocked race.

Some voters already have mail-in ballots in Pennsylvania, the biggest prize on election day among battleground states.

-with AAP