Before heading there, she indirectly criticised former President Donald Trump, suggesting her opponent in the November 5 election is a "coward" whose politics focused on putting down rivals.

The remarks came in a campaign appearance in the critical battleground state of Pennsylvania with running mate Minnesota Governor Tim Walz, before Harris heads to the Democratic National Convention in Chicago, which kicks off on Monday.

"Over the last several years, there's been this kind of perversion that has taken place, I think, which suggests that the measure of the strength of a leader is based on who you beat down. When what we know is the real and true measure of the strength of a leader is based on who you lift up," Harris told a crowd of supporters.

"Anybody who's about beating down other people is a coward."

She did not directly name Trump, who, in a campaign appearance on Saturday in eastern Pennsylvania, referred to Harris as a "radical" and a "lunatic."

Opinion polls have shown Harris bringing fresh energy to the campaign and closing the gap with Trump both nationally and in many of the eight highly competitive states, including Pennsylvania, that will play a decisive role in picking Democratic President Joe Biden's successor.

She said she had nearly finished writing the speech she would deliver to accept the Democratic presidential nomination on Thursday.

"There will be a lot about what I believe is a way forward, a new way forward, and bringing everyone along in that," she told reporters outside a restaurant.