The location suggests Pennsylvania Governor Josh Shapiro has moved to the top of a short list of potential running mates and the Harris campaign has decided the state that Democrats won back from Republicans in 2020 is a must-win once again.

Typically, campaigns begin thinking about their vice-presidential pick after the primary race ends in the spring, giving them months to vet candidates and make a decision on who the candidate meshes with best personally and politically.

Harris is being forced to select her running mate on a highly compressed timeline.

She faces an August 7 deadline set by the Democratic National Committee, but the decision is likely to come sooner, according to the sources.

The candidates are informally auditioning for the job, hitting the airwaves and campaign stops to showcase what they would bring to the Harris ticket.

The short list of candidates under consideration include Shapiro, Kentucky Governor Andy Beshear, US Senator Mark Kelly of Arizona, Minnesota Governor Tim Walz and Secretary of Transportation Pete Buttigieg.

Shapiro, for example, delivered a rousing testimonial about Harris in the Philadelphia suburbs on Monday.

"She's not only ready, she's damned ready," he said to a cheering crowd.

"And you know who else knows she's ready? Donald Trump knows she's ready."

Walz, the governor of Minnesota, is credited with pushing the Democrats' new criticism of Trump and his running mate JD Vance - that they are "weird."

"The fascists depend on us going back, but we're not afraid of weird people," Walz said on Saturday of the Republican ticket.

"We're a little bit creeped out, but we're not afraid."