Before their convention that this week will signal the final sprint to November, Vice President Kamala Harris and her running mate, Gov. Tim Walz of Minnesota, headed out on a brief bus tour on Sunday to fire up voters in perhaps the most crucial battleground state in the 2024 election.

As they toured western Pennsylvania, their play for support beyond the state’s more liberal cities was apparent at the team’s first stop, a field office in Rochester, Pa., in the largely conservative Beaver County: Ms. Harris picked up a volunteer’s cellphone to speak with a resident from Erie, a northwestern city in one of the state’s swingiest counties, which Hillary Clinton lost in 2016 but Joseph R. Biden Jr. won four years later.

“I love Erie,” Ms. Harris said. “At some point we’ll get to Erie.”

Ms. Harris and Mr. Walz were joined on the outing by their spouses, Doug Emhoff and Gwen Walz, traveling in two new campaign buses from the Pittsburgh airport, where they arrived on Air Force Two to greet a small group of supporters.

The Pittsburgh and Philadelphia areas are the two main drivers of Democratic support in Pennsylvania, a state whose 19 electoral votes could decide the presidency. Recent polling shows a neck-and-neck race there between Ms. Harris and former President Donald J. Trump, with some surveys showing Ms. Harris gaining a narrow edge recently.

Mr. Trump is also increasing his presence in Pennsylvania — on Saturday he held a rally in Wilkes-Barre and another is set in York on Monday, while Senator JD Vance of Ohio, his running mate, campaigns in Philadelphia.

Both candidates have used their trips in the state to make attacks.

Speaking to a crowd of supporters outside the Rochester campaign office on Sunday, Ms. Harris appeared to suggest that Mr. Trump was a “coward” — just a day after he had called her a “radical” and a “lunatic” in Wilkes-Barre.

“Over the last several years there’s been this kind of perversion that has taken place, I think, which is to suggest that the measure of the strength of a leader is based on who you beat down,” Ms. Harris said — though she did not name Mr. Trump. “Anybody who’s about beating down other people is a coward.”

President Biden often preferred to campaign in Philadelphia, by far the state’s largest metro area. The city is easily reached from Washington and close to his home state of Delaware, nestled in friendly territory surrounded by counties he carried in 2020.

Outside Allegheny County, where Pittsburgh sits, western Pennsylvania is less hospitable to Democrats, and Ms. Harris’s visit there suggests she may branch out more than Mr. Biden did.

“God bless them, they need to,” Nancy Cannon, a retired teacher who showed up at the Pittsburgh airport, said of the campaign’s planned visits. She described a landscape of Trump supporters, where she knew of Democrats who were afraid to put out their own lawn signs. “Maybe they would feel more supported,” she said, if Ms. Harris and Mr. Walz showed up.

Ms. Cannon said Ms. Harris’s swift ascent to the top of the Democratic ticket had given a jolt of energy to her family and friends. “I love Joe. But I am happier to see a younger person. I feel like this is an Obama moment.”

The trip points to the Democratic hopes that Mr. Walz can help the party reach working-class voters outside the big cities. The suburbs and small towns surrounding Pittsburgh resemble areas in Minnesota where Mr. Walz has performed well in his previous races.

One campaign stop on Sunday was also a clear effort to tap into Mr. Walz’s personal history: a football practice at Aliquippa High School in a former steel town with many Black residents that has struggled economically for decades. Mr. Walz’s own experience as a high school football coach has been a touchstone of the campaign so far, with Ms. Harris occasionally referring to him as “Coach Walz.”

Mr. Walz told the team he “remembered every single call” from his team’s state football championship. He and Ms. Harris were joined at the event by Jerome Bettis, the Hall of Fame running back who won a Super Bowl with the Pittsburgh Steelers. Mr. Bettis, nicknamed The Bus, was a surprise, if appropriately named, addition to the Harris-Walz motor coach tour, which also made short stops at a nearby firehouse, a Sheetz gas station and Primanti Bros., a beloved chain of sandwich shops founded in Pittsburgh.

“The way you win elections in Pennsylvania is literally going everywhere,” said Lt. Gov. Austin Davis of Pennsylvania, a Democrat who is from the Pittsburgh area. “Philadelphia, Pittsburgh, but all the small communities in between.”

Mr. Davis also said that Ms. Harris would find a welcome audience in the working-class region for the economic agenda she rolled out last week that focused on lowering the costs of everyday life and cutting taxes for working families.

Conor Lamb, a Democrat who once represented a congressional district outside Pittsburgh, said Mr. Biden had become well known to many Pennsylvania voters, thanks to his decades of service as a senator in neighboring Delaware, an advantage Ms. Harris does not possess.

Mr. Lamb said that the bus tour reflected the fact that Democrats had learned their lesson from Mrs. Clinton losing the state after focusing most of her campaign on the major cities.

“You can’t just do Philly and Pittsburgh and rely on social media,” Mr. Lamb said.

And he agreed that Ms. Harris’s economic plans would be popular in places like Rochester, a small town once known for its glassmaking factories that sits in his old district.

“Some of the things that she laid out in that speech — the child tax credit, extending health care subsidies — all that kind of stuff is just universal,” Mr. Lamb said. “That should do as well in Rochester as it does in the heart of the city of Pittsburgh. And I’ve felt in the past that Democrats were a little bit afraid to go on the offense in those areas.”