US Vice President Kamala Harris has embarked on a campaign bus tour across the eastern battleground state of Pennsylvania to win over voters in the November presidential election.

Harris visited Pittsburgh in western Pennsylvania on Sunday with her running mate, Minnesota Governor Tim Walz.

She shook hands and took photos with her supporters at an airport before boarding a bus bearing the names of Harris and Walz. She pledged in a speech that her team would fight for freedom and the middle class.

Harris held her first joint rally with Walz in Pennsylvania on August 6. Analysts say she has been seeking to show her eagerness to win the state in the election.

Former President Donald Trump, the Republican candidate, also stumped in Pennsylvania on Saturday.

The Democratic Party is due to open a national convention in Chicago in the Midwestern state of Illinois on Monday.